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The Decreasing Age at Menarche in Mexico
Accepted Manuscript The decreasing age at menarche in Mexico Ma. Luisa Marván, Rosa Lilia Catillo-López, Verónica Alcalá-Herrera, Diana del Callejo PII:
S1083-3188(16)00181-9
DOI:
10.1016/j.jpag.2016.02.006
Reference:
PEDADO 1966
To appear in:
Journal of Pediatric and Adolescent Gynecology
Received Date: 17 November 2015 Revised Date:
8 February 2016
Accepted Date: 14 February 2016
Please cite this article as: Marván ML, Catillo-López RL, Alcalá-Herrera V, del Callejo D, The decreasing age at menarche in Mexico, Journal of Pediatric and Adolescent Gynecology (2016), doi: 10.1016/ j.jpag.2016.02.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
The decreasing age at menarche in Mexico
del Callejo3
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Ma. Luisa Marván1, Rosa Lilia Catillo-López1, Verónica Alcalá-Herrera2 and Diana
1
Instituto de Investigaciones Psicológicas, Universidad Veracruzana. México
2
Facultad de Psicología, Universidad Nacional Autónoma de México. México
3
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Instituto de Investigaciones y Estudios Superiores Económicos y Sociales, Universidad Veracruzana, México
Disclosure statement: No potential conflict of interest was reported by the authors.
Short title: Age at menarche in Mexico
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Key words: menarche, secular trend, Mexico, puberty
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Corresponding author: Ma. Luisa Marván Instituto de Investigaciones Psicológicas Universidad Veracruzana Av. Dr. Luis Castelazo Ayala s/n Col. Industrial Ánimas Xalapa, Ver. 91190, MEXICO Tel: +52 (228)841-89-00 ext. 13210 E-mail: [email protected]
ACCEPTED MANUSCRIPT 1 The decreasing age at menarche in Mexico
Abstract: Objectives: To estimate the current age at menarche in two urban areas
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from Mexico and to analyze secular trend in the age at menarche in Mexico City. Design: Cross-sectional survey and secondary data analyses.
Settings: a) Samples of female students from Mexico City and from the city
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of Xalapa. b) National health surveys.
Participants: a) Pre and postmenarcheal students, aged 8-17 years b) Data
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for women born between 1904 and 1999 who grew up in Mexico City were obtained from three relevant national health surveys.
Intervention: a) Students were asked if they had had their first period and when it occurred. b) Information on year of birth and age at menarche for the
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remaining of women was obtained from the national surveys; data were divided in groups according to the decade in which the women were born. Results: a) The estimated mean age at menarche was 11.40 and 11.34
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years in Mexico City and Xalapa respectively. b) There were significant differences in age at menarche among women who were born in the different decades
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analyzed. Women who were born in the 1990s or later reported an earlier age at menarche than those who were born from the 1940s to 1980s (p<.0001). Conclusions: These results could be helpful to health and education
professionals who should take into account that since there is currently a widening gap between the physical and psychosocial maturity of girls, they may need special support. Key Words: menarche, secular trend, Mexico, puberty
ACCEPTED MANUSCRIPT 2 Introduction: Menarche is considered an important indicator of pubertal development in girls. The age that menarche occurs is of clinical, public health and social
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importance1 and it reflects numerous health aspects of a population2.
Age at menarche varies substantially between women across different cultures. Rigorous comparisons between the results of different studies are
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complicated due to methodological aspects. However, there are at least two studies in which authors discussed the published data of menarcheal age in
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different countries at the end of the last century or in the first two years of this century. In both studies it was observed that the earliest age of menarche was 12.0 and the latest was 16.1 or 16.2 years3,4. The age at menarche appeared earlier in countries where individuals experience the longest life expectancies, and it was
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associated with good nutritional conditions3.
Age at menarche in Mexico has been reported in some studies conducted between the 1960s and 1990s. In those studies, the age at menarche varied
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between 12.06 and 12.92 in urban areas5–10. In Mexico City, one of the cities in which the current study was conducted, the ages varied between 12.30 and
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12.925,6,8,9.
Several studies worldwide have shown a secular trend towards a reduction
in the age at menarche. However, the magnitude of the trend remains in doubt, since there is no consistency in all studies12. In Europe, the ages at menarche decreased from 16-17 years in the mid19th century to 13 years in the mid-20th century2,13. Subsequent reports also have indicated that age at menarche declined overall for the most part of the 20th century
ACCEPTED MANUSCRIPT 3 in several countries such as: Canada12, Spain14, Korea15,16, India17, Japan18, Taiwan19, Russia20, Turkey 21, and Indonesia22. Whether the secular trends in age at menarche are continuing or have
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reached their limit is still a matter of debate15. According some authors, this trend tends to slow down or stabilize in developed nations11,18, whereas recent studies have shown that menarche is occurring earlier in some countries like China13,
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South Korea16, and Brazil23. In the Netherlands and in the United States, this trend depends on the race/ethnicity and socioeconomic position of girls1,24.
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In Mexico, a reduction in the age at menarche from 1978 to 2000 was reported in an indigenous rural community25 but, as far as we know, no study has been made of possible secular trends in urban non-indigenous populations. There are three methods for obtaining age at menarche: a) The prospective
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method, in which premenarcheal girls are followed regularly and asked each time if they have begun to menstruate. Despite its accuracy, it is not easy to carry out longitudinal studies and therefore, most authors use other methods. b) The status
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quo method in which a girl (or her parent) is asked whether she has had her first menses and her birth date. c) The recall or retrospective method in which post-
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menarcheal women are asking to recall their age at first menses. The objectives of this study were: a) To estimate the current age at
menarche in two urban areas from Mexico, using status quo and retrospective methods; and b) To analyze secular trend in the age at menarche.
Methodology:
ACCEPTED MANUSCRIPT 4 The study was approved by the corresponding board of the Institute of
Current estimated age at menarche:
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Psychological Research, Universidad Veracruzana.
Participants: This study included a non-probabilistic sample composed of 1,823 female students from public schools in Mexico City (the capital of the
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country) or in the city of Xalapa (the capital of the state of Veracruz). Their ages varied between 8 and 17 years and they were attending from fourth grade of
grades in the USA (see Table 1).
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elementary school to first grade of high school, equivalent to fourth through tenth
Procedure: Appointments were made with the principals of schools to explain the objective of the study and ask their permission to conduct it. The school
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boards of 11 schools in Mexico City and seven in Xalapa agreed. The project was then explained to teachers, and a letter was sent to the parents of all female students of the grades mentioned above explaining the nature of the study and
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asking their consent for their daughters' participation. Teachers collected the consents and then asked students who had the consent from their parents if they
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wanted to be part of a research project about adolescence. Teachers stressed that participation was voluntary and that the results would be used exclusively for the research project.
In order to survey the students, two female researchers went to the schools
between January and March 2013 in Mexico City, and between September and December 2014 in Xalapa. When the researchers arrived at the schools, the students were in a designated classroom in which the survey would take place.
ACCEPTED MANUSCRIPT 5 They were asked to respond to the survey as part of a more extensive research project about adolescence in which they were asked if they had had their first period. If they answered affirmatively, they were asked to remember either how old
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they were in years and months when it occurred, or to remember in which year and in which month it happened.
Those students who reported having had some hormonal medical condition
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were excluded.
Data analyses: We conducted probit analyses with 95% confidence intervals
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(CI) in order to estimate the age at menarche using the status quo method. After that, we used descriptive statistics to calculate the mean age at menarche using the retrospective method. Since most of the participants were not able to remember the date of their menarche in years and months, we considered only the
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whole years.
Secular trend of the age at menarche:
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Data collection: Data from 4,655 women who were born between 1904 and 1999 and who grew up in Mexico City were obtained. We selected this city
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because, as the capital of the country, most of the older studies cited in the Introduction section had been conducted there. First, some data were obtained by directly asking women to state their date
of birth and to recall the age of their first menses (in whole years). These participants were contacted through personal networks by snowball using social media. Second, the following national surveys were consulted to obtain similar data from other women: Encuesta Nacional de Salud, 2000 (National Health Survey),
ACCEPTED MANUSCRIPT 6 Encuesta Nacional de Salud Reproductiva, 2003 (National Reproductive Health Survey), and Encuesta Nacional de Salud y Nutrición, 2006 (National Health and Nutrition Survey). Only data from Mexico City were taken; information on age at
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menarche as whole years was complete for all women in the data set. The year of birth variable was used to identify birth cohorts and analyze a secular trend.
Finally, some data from participants who were born in the 1990s were taken
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from the sample surveyed in schools described above in the previous section of the Methodology.
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Data analyses: Data were divided in seven groups according the decade in which the women were born. The mean age at menarche of each group was calculated. A Kruskal-Wallis test with comparisons of mean ranks was conducted
Results:
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to compare the calculated age at menarche among groups.
Current estimated age at menarche:
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In both the Mexico City and Xalapa samples, the distribution of data was normal and the chi-square values indicated a good fit to accept the regression
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models. Distribution of girls by age at menarche is summarized in Table 2. According to these models, in Mexico City the estimated mean age at
menarche was 11.40 years (CI 11.14 – 11.63) using the status quo method. In the same sample of adolescents, the calculated mean age using the retrospective method was 11.42 years (CI 11.34 – 11.50). The 95% Cls for the ages at menarche for retrospective and status quo data were overlapped.
ACCEPTED MANUSCRIPT 7 In Xalapa, when the status quo procedure was used, the estimated mean age at menarche was 11.34 years (CI 11.16 – 11.50), and when the retrospective method was used, the calculated mean age was 11.45 (CI 11.36 – 11.54). The
Secular trend of the age at menarche in Mexico City:
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95% Cls for the ages for retrospective and status quo data were also overlapped.
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Recalled ages at menarche by decade of birth are shown in Table 3.
A long-term trend of decreasing age of menarche was observed (x2= 385.39, df=5,
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p<.0001).
Women who were born in the 1950s reported a lower age at menarche than those who were born in the 1940s or earlier (p<.0001). In subsequent years, the age at menarche remained stable from the 1950s through the 1970s but there was
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another significant decline in age for women who were born in the 1980s (p<.0001) and finally, there was again a significant decrease in age for women born in the
Discussion:
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1990s (p<.0001).
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The results of this study are consistent with previous reports showing a trend toward younger ages for menarche. The age in Mexico City seems to be considerably earlier than ages reported in previous studies conducted in the 1960s-1990s in the same city that also used the status quo method. Because data analyses of the oldest studies did not contemplate regression models, Malina and colleagues7 re-analyzed the data from these studies in order to estimate the age at menarche using probit analyses, thus making the ages found more comparable
ACCEPTED MANUSCRIPT 8 with recent data. In our study, age at menarche was 11.40 years, which is much lower than those reported in the studies conducted with women who were born between the 1940s and 1980s (from 12.64 to 12.75 years) 7,9.
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When we collected the age at menarche from women of different ages and examined it by decade of birth, we found a significant decrease in the age of
menarche over the long-term. This resultant secular trend ratifies a reduction in
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age at menarche among women from the capital of the country, which is consistent
the Introduction section13,16,23.
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with other recent studies conducted in different countries that were mentioned in
The causes of these changes are not fully understood, but some possible explanations are: 1) An increasingly lengthy life expectancy which is associated with general improvement in the health of the population. In fact, the fastest rates
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of decline in the age at menarche have been observed in countries with greater economic transformations2. In Mexico, life expectancy varied between 57 and 61 years in the1960s and has been steadily increasing until reaching 77 years during
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the last five years26. 2) An increasing prevalence of childhood obesity12. In Mexico, the percentage of girls (5-11 years old) with either overweight or obesity increased
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33% from 1999 to 2006. This increase was observed in children from all socioeconomic classes and regions of the country27. These numbers have neither increased nor decreased in recent years28, and currently Mexico holds first place in childhood obesity worldwide29. 3) A secular trend of increasing height25. In Mexico, there was a significant increase in the height of girls from 1999 to 200627. 4) A higher exposure to environmental estrogens or endocrine disrupting compounds12,14. 5) Changes in family structure. Previous studies have shown that
ACCEPTED MANUSCRIPT 9 menarche occurs at an earlier age among girls raised in stressful family circumstances, or in the absence of the girl’s father30,31. In Mexico, it is becoming more and more common for children to grow up in families where no father is
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present, either because of divorce or by having a single mother.
The trend towards earlier ages at menarche may have multiple health
implications that are important to take into consideration. Menarche at an early age
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has been associated with abdominal type obesity, insulin resistance, glucose
intolerance, cardiovascular risk, coronary heart disease, increased bone mineral
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density, and an increased rate of cancer mortality, especially breast cancer11. Moreover, because the secular trend in decline in the age at menarche has coincided with later ages of psychosocial maturity, physical maturity outpaces cognitive and emotional development. This gap between physical and psychosocial
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maturity may cause girls to engage in risky behaviors with both individual and societal costs32,33. Moreover, the earlier onset of sexual activity that has been associated with the decrease in the age at menarche32, especially when is linked to
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the delay in psychosocial maturity, may lead to unwanted pregnancies. Unfortunately, a recent report indicates that Mexico has the highest rate of
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adolescent mothers among countries belonging to the Organization for Economic Co-operation and Development (OECD)34. Additionally, the increasing time elapsed between the onset of sexual activity and marriage/motherhood may lead an increase in the number of sexual partners, with the consequent risk of getting sexually transmitted diseases. Finally, analyzing data from girls, we found that the estimated ages at menarche using the status quo method were similar to the calculated using the
ACCEPTED MANUSCRIPT 10 retrospective method, indicating that although the retrospective method is less accurate, adolescents have clear memories of their menarche. This result is not surprising since Must and colleagues35 demonstrated that there is a strong
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correlation between age at menarche of girls, calculated using the prospective
method, and the age recalled by the same women 30 years later. Thus, memories of menarche persist because it is a significant event with a profound impact on
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women’s lives36, and recall ability is directly related to the importance the event for the respondent37.
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The findings of this study must be interpreted in the context of its limitations. First, there was a selection bias for students from urban areas and therefore adolescents who do not go to school or who live in rural areas were not included. Further studies with these samples are needed to generalize results. Moreover, the
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students’ parents were not asked about their daughters’ age of menarche to confirm the accuracy of the students’ answers. Finally, we did not examine the nutritional state, body mass index or other variables that affect the menarcheal
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age.
Despite these cautions, our results could be useful for health professionals
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in order to implement preventive actions to decrease the multiple physical and psychosocial problems associated with the earlier ages at menarche. Moreover, education professionals should be conscious that decreasing age at menarche requires changes in elementary schools’ educational programs, mainly the sexual education programs. Finally, because it is well known that when a girl reaches puberty, her biological processes interact with the social context to affect her individual emotional and social development33, it is important to be sensitive to any
ACCEPTED MANUSCRIPT 11 gap between a girl’s physical and psychosocial maturity in order to evaluate her behavior and offer proactive guidance if required.
1.
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ACCEPTED MANUSCRIPT 1 Table 1. Number (and percent) of participants who were surveyed by the status
Xalapa n (%)
8
23 (2.1%)
9 (1.3%)
9
72 (6.4%)
69 (9.8%)
10
91 (8.1%)
62 (8.8%)
11
100 (8.9%)
73 (10.4%)
12
149 (13.3%)
149 (21.2%)
13
201 (17.9%)
14
248 (22.1%)
15
106 (9.5%)
16
105 (9.4%)
17
26 (2.3%)
29 (4.1%)
286 (25.5%)
213 (30.3%)
100 (14.2%) 116 (16.5%) 44 (6.3%) 51 (7.3%)
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835 (74.5%)
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4th - 6th (elementary school) 7th - 10th (medium and high school)
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Grade
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Age
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Mexico City n (%)
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quo method.
489 (67.7%)
ACCEPTED MANUSCRIPT 2 Table 2. Frequency (and percentage) of girls who had attained menarche by age. Mexico City
Xalapa
Frequency
Percentage
8
0
0%
Probability of occurrence .004
9
2
2.78%
.032
2
10
11
12.09%
.141
9
11
36
36.00%
.379
22
12
104
68.80%
.678
113
75.84%
.713
13
186
92.54%
.891
91
91.00%
.920
14
240
96.77%
15
104
98.11%
16
105
100%
17
26
100%
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0
2.90%
.024
14.52%
.130
30.14%
.389
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SC
0
Probability of occurrence .002
.977
114
98.28%
.988
.997
44
100%
.999
1.00
51
100%
1.00
1.00
29
100%
1.00
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Mexico City: x2 = 14.01 (n.s.) Xalapa: x2 = 4.50 (n.s.)
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Frequency Percentage
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Age
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Table 3. Mean age at menarche calculated (retrospective) and estimated (status
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quo) for women from Mexico City.
n
Mean
CI
1940s or earlier
565
12.95
12.82-13.08
1950s
693
12.58
12.46-12.70
1960s
859
12.52
1970s
864
1980s
902
1990s
772
Status quo (1990s – 2000s)
1121
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12.42-12.62
12.52
12.42-12.63
12.20
12.11-12.29
11.52
11.44-11.60
11.40
11.14 – 11.63
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Decade of birth
S1083-3188(16)00181-9
DOI:
10.1016/j.jpag.2016.02.006
Reference:
PEDADO 1966
To appear in:
Journal of Pediatric and Adolescent Gynecology
Received Date: 17 November 2015 Revised Date:
8 February 2016
Accepted Date: 14 February 2016
Please cite this article as: Marván ML, Catillo-López RL, Alcalá-Herrera V, del Callejo D, The decreasing age at menarche in Mexico, Journal of Pediatric and Adolescent Gynecology (2016), doi: 10.1016/ j.jpag.2016.02.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
The decreasing age at menarche in Mexico
del Callejo3
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Ma. Luisa Marván1, Rosa Lilia Catillo-López1, Verónica Alcalá-Herrera2 and Diana
1
Instituto de Investigaciones Psicológicas, Universidad Veracruzana. México
2
Facultad de Psicología, Universidad Nacional Autónoma de México. México
3
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Instituto de Investigaciones y Estudios Superiores Económicos y Sociales, Universidad Veracruzana, México
Disclosure statement: No potential conflict of interest was reported by the authors.
Short title: Age at menarche in Mexico
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Key words: menarche, secular trend, Mexico, puberty
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Corresponding author: Ma. Luisa Marván Instituto de Investigaciones Psicológicas Universidad Veracruzana Av. Dr. Luis Castelazo Ayala s/n Col. Industrial Ánimas Xalapa, Ver. 91190, MEXICO Tel: +52 (228)841-89-00 ext. 13210 E-mail: [email protected]
ACCEPTED MANUSCRIPT 1 The decreasing age at menarche in Mexico
Abstract: Objectives: To estimate the current age at menarche in two urban areas
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from Mexico and to analyze secular trend in the age at menarche in Mexico City. Design: Cross-sectional survey and secondary data analyses.
Settings: a) Samples of female students from Mexico City and from the city
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of Xalapa. b) National health surveys.
Participants: a) Pre and postmenarcheal students, aged 8-17 years b) Data
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for women born between 1904 and 1999 who grew up in Mexico City were obtained from three relevant national health surveys.
Intervention: a) Students were asked if they had had their first period and when it occurred. b) Information on year of birth and age at menarche for the
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remaining of women was obtained from the national surveys; data were divided in groups according to the decade in which the women were born. Results: a) The estimated mean age at menarche was 11.40 and 11.34
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years in Mexico City and Xalapa respectively. b) There were significant differences in age at menarche among women who were born in the different decades
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analyzed. Women who were born in the 1990s or later reported an earlier age at menarche than those who were born from the 1940s to 1980s (p<.0001). Conclusions: These results could be helpful to health and education
professionals who should take into account that since there is currently a widening gap between the physical and psychosocial maturity of girls, they may need special support. Key Words: menarche, secular trend, Mexico, puberty
ACCEPTED MANUSCRIPT 2 Introduction: Menarche is considered an important indicator of pubertal development in girls. The age that menarche occurs is of clinical, public health and social
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importance1 and it reflects numerous health aspects of a population2.
Age at menarche varies substantially between women across different cultures. Rigorous comparisons between the results of different studies are
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complicated due to methodological aspects. However, there are at least two studies in which authors discussed the published data of menarcheal age in
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different countries at the end of the last century or in the first two years of this century. In both studies it was observed that the earliest age of menarche was 12.0 and the latest was 16.1 or 16.2 years3,4. The age at menarche appeared earlier in countries where individuals experience the longest life expectancies, and it was
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associated with good nutritional conditions3.
Age at menarche in Mexico has been reported in some studies conducted between the 1960s and 1990s. In those studies, the age at menarche varied
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between 12.06 and 12.92 in urban areas5–10. In Mexico City, one of the cities in which the current study was conducted, the ages varied between 12.30 and
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12.925,6,8,9.
Several studies worldwide have shown a secular trend towards a reduction
in the age at menarche. However, the magnitude of the trend remains in doubt, since there is no consistency in all studies12. In Europe, the ages at menarche decreased from 16-17 years in the mid19th century to 13 years in the mid-20th century2,13. Subsequent reports also have indicated that age at menarche declined overall for the most part of the 20th century
ACCEPTED MANUSCRIPT 3 in several countries such as: Canada12, Spain14, Korea15,16, India17, Japan18, Taiwan19, Russia20, Turkey 21, and Indonesia22. Whether the secular trends in age at menarche are continuing or have
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reached their limit is still a matter of debate15. According some authors, this trend tends to slow down or stabilize in developed nations11,18, whereas recent studies have shown that menarche is occurring earlier in some countries like China13,
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South Korea16, and Brazil23. In the Netherlands and in the United States, this trend depends on the race/ethnicity and socioeconomic position of girls1,24.
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In Mexico, a reduction in the age at menarche from 1978 to 2000 was reported in an indigenous rural community25 but, as far as we know, no study has been made of possible secular trends in urban non-indigenous populations. There are three methods for obtaining age at menarche: a) The prospective
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method, in which premenarcheal girls are followed regularly and asked each time if they have begun to menstruate. Despite its accuracy, it is not easy to carry out longitudinal studies and therefore, most authors use other methods. b) The status
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quo method in which a girl (or her parent) is asked whether she has had her first menses and her birth date. c) The recall or retrospective method in which post-
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menarcheal women are asking to recall their age at first menses. The objectives of this study were: a) To estimate the current age at
menarche in two urban areas from Mexico, using status quo and retrospective methods; and b) To analyze secular trend in the age at menarche.
Methodology:
ACCEPTED MANUSCRIPT 4 The study was approved by the corresponding board of the Institute of
Current estimated age at menarche:
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Psychological Research, Universidad Veracruzana.
Participants: This study included a non-probabilistic sample composed of 1,823 female students from public schools in Mexico City (the capital of the
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country) or in the city of Xalapa (the capital of the state of Veracruz). Their ages varied between 8 and 17 years and they were attending from fourth grade of
grades in the USA (see Table 1).
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elementary school to first grade of high school, equivalent to fourth through tenth
Procedure: Appointments were made with the principals of schools to explain the objective of the study and ask their permission to conduct it. The school
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boards of 11 schools in Mexico City and seven in Xalapa agreed. The project was then explained to teachers, and a letter was sent to the parents of all female students of the grades mentioned above explaining the nature of the study and
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asking their consent for their daughters' participation. Teachers collected the consents and then asked students who had the consent from their parents if they
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wanted to be part of a research project about adolescence. Teachers stressed that participation was voluntary and that the results would be used exclusively for the research project.
In order to survey the students, two female researchers went to the schools
between January and March 2013 in Mexico City, and between September and December 2014 in Xalapa. When the researchers arrived at the schools, the students were in a designated classroom in which the survey would take place.
ACCEPTED MANUSCRIPT 5 They were asked to respond to the survey as part of a more extensive research project about adolescence in which they were asked if they had had their first period. If they answered affirmatively, they were asked to remember either how old
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they were in years and months when it occurred, or to remember in which year and in which month it happened.
Those students who reported having had some hormonal medical condition
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were excluded.
Data analyses: We conducted probit analyses with 95% confidence intervals
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(CI) in order to estimate the age at menarche using the status quo method. After that, we used descriptive statistics to calculate the mean age at menarche using the retrospective method. Since most of the participants were not able to remember the date of their menarche in years and months, we considered only the
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whole years.
Secular trend of the age at menarche:
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Data collection: Data from 4,655 women who were born between 1904 and 1999 and who grew up in Mexico City were obtained. We selected this city
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because, as the capital of the country, most of the older studies cited in the Introduction section had been conducted there. First, some data were obtained by directly asking women to state their date
of birth and to recall the age of their first menses (in whole years). These participants were contacted through personal networks by snowball using social media. Second, the following national surveys were consulted to obtain similar data from other women: Encuesta Nacional de Salud, 2000 (National Health Survey),
ACCEPTED MANUSCRIPT 6 Encuesta Nacional de Salud Reproductiva, 2003 (National Reproductive Health Survey), and Encuesta Nacional de Salud y Nutrición, 2006 (National Health and Nutrition Survey). Only data from Mexico City were taken; information on age at
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menarche as whole years was complete for all women in the data set. The year of birth variable was used to identify birth cohorts and analyze a secular trend.
Finally, some data from participants who were born in the 1990s were taken
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from the sample surveyed in schools described above in the previous section of the Methodology.
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Data analyses: Data were divided in seven groups according the decade in which the women were born. The mean age at menarche of each group was calculated. A Kruskal-Wallis test with comparisons of mean ranks was conducted
Results:
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to compare the calculated age at menarche among groups.
Current estimated age at menarche:
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In both the Mexico City and Xalapa samples, the distribution of data was normal and the chi-square values indicated a good fit to accept the regression
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models. Distribution of girls by age at menarche is summarized in Table 2. According to these models, in Mexico City the estimated mean age at
menarche was 11.40 years (CI 11.14 – 11.63) using the status quo method. In the same sample of adolescents, the calculated mean age using the retrospective method was 11.42 years (CI 11.34 – 11.50). The 95% Cls for the ages at menarche for retrospective and status quo data were overlapped.
ACCEPTED MANUSCRIPT 7 In Xalapa, when the status quo procedure was used, the estimated mean age at menarche was 11.34 years (CI 11.16 – 11.50), and when the retrospective method was used, the calculated mean age was 11.45 (CI 11.36 – 11.54). The
Secular trend of the age at menarche in Mexico City:
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95% Cls for the ages for retrospective and status quo data were also overlapped.
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Recalled ages at menarche by decade of birth are shown in Table 3.
A long-term trend of decreasing age of menarche was observed (x2= 385.39, df=5,
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p<.0001).
Women who were born in the 1950s reported a lower age at menarche than those who were born in the 1940s or earlier (p<.0001). In subsequent years, the age at menarche remained stable from the 1950s through the 1970s but there was
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another significant decline in age for women who were born in the 1980s (p<.0001) and finally, there was again a significant decrease in age for women born in the
Discussion:
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1990s (p<.0001).
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The results of this study are consistent with previous reports showing a trend toward younger ages for menarche. The age in Mexico City seems to be considerably earlier than ages reported in previous studies conducted in the 1960s-1990s in the same city that also used the status quo method. Because data analyses of the oldest studies did not contemplate regression models, Malina and colleagues7 re-analyzed the data from these studies in order to estimate the age at menarche using probit analyses, thus making the ages found more comparable
ACCEPTED MANUSCRIPT 8 with recent data. In our study, age at menarche was 11.40 years, which is much lower than those reported in the studies conducted with women who were born between the 1940s and 1980s (from 12.64 to 12.75 years) 7,9.
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When we collected the age at menarche from women of different ages and examined it by decade of birth, we found a significant decrease in the age of
menarche over the long-term. This resultant secular trend ratifies a reduction in
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age at menarche among women from the capital of the country, which is consistent
the Introduction section13,16,23.
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with other recent studies conducted in different countries that were mentioned in
The causes of these changes are not fully understood, but some possible explanations are: 1) An increasingly lengthy life expectancy which is associated with general improvement in the health of the population. In fact, the fastest rates
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of decline in the age at menarche have been observed in countries with greater economic transformations2. In Mexico, life expectancy varied between 57 and 61 years in the1960s and has been steadily increasing until reaching 77 years during
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the last five years26. 2) An increasing prevalence of childhood obesity12. In Mexico, the percentage of girls (5-11 years old) with either overweight or obesity increased
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33% from 1999 to 2006. This increase was observed in children from all socioeconomic classes and regions of the country27. These numbers have neither increased nor decreased in recent years28, and currently Mexico holds first place in childhood obesity worldwide29. 3) A secular trend of increasing height25. In Mexico, there was a significant increase in the height of girls from 1999 to 200627. 4) A higher exposure to environmental estrogens or endocrine disrupting compounds12,14. 5) Changes in family structure. Previous studies have shown that
ACCEPTED MANUSCRIPT 9 menarche occurs at an earlier age among girls raised in stressful family circumstances, or in the absence of the girl’s father30,31. In Mexico, it is becoming more and more common for children to grow up in families where no father is
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present, either because of divorce or by having a single mother.
The trend towards earlier ages at menarche may have multiple health
implications that are important to take into consideration. Menarche at an early age
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has been associated with abdominal type obesity, insulin resistance, glucose
intolerance, cardiovascular risk, coronary heart disease, increased bone mineral
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density, and an increased rate of cancer mortality, especially breast cancer11. Moreover, because the secular trend in decline in the age at menarche has coincided with later ages of psychosocial maturity, physical maturity outpaces cognitive and emotional development. This gap between physical and psychosocial
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maturity may cause girls to engage in risky behaviors with both individual and societal costs32,33. Moreover, the earlier onset of sexual activity that has been associated with the decrease in the age at menarche32, especially when is linked to
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the delay in psychosocial maturity, may lead to unwanted pregnancies. Unfortunately, a recent report indicates that Mexico has the highest rate of
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adolescent mothers among countries belonging to the Organization for Economic Co-operation and Development (OECD)34. Additionally, the increasing time elapsed between the onset of sexual activity and marriage/motherhood may lead an increase in the number of sexual partners, with the consequent risk of getting sexually transmitted diseases. Finally, analyzing data from girls, we found that the estimated ages at menarche using the status quo method were similar to the calculated using the
ACCEPTED MANUSCRIPT 10 retrospective method, indicating that although the retrospective method is less accurate, adolescents have clear memories of their menarche. This result is not surprising since Must and colleagues35 demonstrated that there is a strong
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correlation between age at menarche of girls, calculated using the prospective
method, and the age recalled by the same women 30 years later. Thus, memories of menarche persist because it is a significant event with a profound impact on
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women’s lives36, and recall ability is directly related to the importance the event for the respondent37.
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The findings of this study must be interpreted in the context of its limitations. First, there was a selection bias for students from urban areas and therefore adolescents who do not go to school or who live in rural areas were not included. Further studies with these samples are needed to generalize results. Moreover, the
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students’ parents were not asked about their daughters’ age of menarche to confirm the accuracy of the students’ answers. Finally, we did not examine the nutritional state, body mass index or other variables that affect the menarcheal
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age.
Despite these cautions, our results could be useful for health professionals
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in order to implement preventive actions to decrease the multiple physical and psychosocial problems associated with the earlier ages at menarche. Moreover, education professionals should be conscious that decreasing age at menarche requires changes in elementary schools’ educational programs, mainly the sexual education programs. Finally, because it is well known that when a girl reaches puberty, her biological processes interact with the social context to affect her individual emotional and social development33, it is important to be sensitive to any
ACCEPTED MANUSCRIPT 11 gap between a girl’s physical and psychosocial maturity in order to evaluate her behavior and offer proactive guidance if required.
1.
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ACCEPTED MANUSCRIPT 1 Table 1. Number (and percent) of participants who were surveyed by the status
Xalapa n (%)
8
23 (2.1%)
9 (1.3%)
9
72 (6.4%)
69 (9.8%)
10
91 (8.1%)
62 (8.8%)
11
100 (8.9%)
73 (10.4%)
12
149 (13.3%)
149 (21.2%)
13
201 (17.9%)
14
248 (22.1%)
15
106 (9.5%)
16
105 (9.4%)
17
26 (2.3%)
29 (4.1%)
286 (25.5%)
213 (30.3%)
100 (14.2%) 116 (16.5%) 44 (6.3%) 51 (7.3%)
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835 (74.5%)
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4th - 6th (elementary school) 7th - 10th (medium and high school)
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Grade
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Age
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Mexico City n (%)
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quo method.
489 (67.7%)
ACCEPTED MANUSCRIPT 2 Table 2. Frequency (and percentage) of girls who had attained menarche by age. Mexico City
Xalapa
Frequency
Percentage
8
0
0%
Probability of occurrence .004
9
2
2.78%
.032
2
10
11
12.09%
.141
9
11
36
36.00%
.379
22
12
104
68.80%
.678
113
75.84%
.713
13
186
92.54%
.891
91
91.00%
.920
14
240
96.77%
15
104
98.11%
16
105
100%
17
26
100%
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0
2.90%
.024
14.52%
.130
30.14%
.389
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0
Probability of occurrence .002
.977
114
98.28%
.988
.997
44
100%
.999
1.00
51
100%
1.00
1.00
29
100%
1.00
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Mexico City: x2 = 14.01 (n.s.) Xalapa: x2 = 4.50 (n.s.)
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Frequency Percentage
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Age
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Table 3. Mean age at menarche calculated (retrospective) and estimated (status
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quo) for women from Mexico City.
n
Mean
CI
1940s or earlier
565
12.95
12.82-13.08
1950s
693
12.58
12.46-12.70
1960s
859
12.52
1970s
864
1980s
902
1990s
772
Status quo (1990s – 2000s)
1121
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12.42-12.62
12.52
12.42-12.63
12.20
12.11-12.29
11.52
11.44-11.60
11.40
11.14 – 11.63
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Decade of birth