Association of Early Menarche with Adolescent Health in the Setting of Rapidly Decreasing Age at Menarche

Journal Pre-proof Association of early menarche with adolescent health in the setting of rapidly decreasing age at menarche Eun Jeong Yu, Seung-Ah Choe, Jae-Won Yun, Mia Son PII:

S1083-3188(19)30375-4

DOI:

https://doi.org/10.1016/j.jpag.2019.12.006

Reference:

PEDADO 2432

To appear in:

Journal of Pediatric and Adolescent Gynecology

Received Date: 16 September 2019 Revised Date:

7 December 2019

Accepted Date: 13 December 2019

Please cite this article as: Yu EJ, Choe S-A, Yun J-W, Son M, Association of early menarche with adolescent health in the setting of rapidly decreasing age at menarche, Journal of Pediatric and Adolescent Gynecology (2020), doi: https://doi.org/10.1016/j.jpag.2019.12.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Inc. on behalf of North American Society for Pediatric and Adolescent Gynecology.

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Title Page

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Title: Association of early menarche with adolescent health in the setting of rapidly

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decreasing age at menarche

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Short Running Title: Early menarche and adolescent health

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Eun Jeong Yu1

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Department of Obstetrics and Gynecology, CHA University, Gyunggi, Korea

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Seung-Ah Choe1,2

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Department of Obstetrics and Gynecology, CHA University, Gyunggi, Korea

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Department of Epidemiology, Brown University, Providence, RI, USA

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Jae-Won Yun3

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Chuncheon, Korea

Department of Preventive Medicine, School of Medicine, Kangwon National University,

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Mia Son3

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Chuncheon, Korea

Department of Preventive Medicine, School of Medicine, Kangwon National University,

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Corresponding author: Seung-Ah Choe

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Department of Obstetrics and Gynecology, CHA University, Gyunggi, Republic of Korea;

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Department of Epidemiology, Brown University, Providence, RI, USA

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E-mail: [email protected]; [email protected]

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Abstract

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Study Objective: This study aimed to investigate the association between age at menarche

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(AAM) and adverse health indicators in adolescent girls.

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Design: A retrospective cohort study

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Setting: Population-based survey data

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Participants: A total of 319,437 female participants aged 12-18 years of the Korea Youth Risk

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Behaviour Web-based Survey

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Interventions and Main Outcome Measures: We assessed associations between AAM

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(categorized as ≤10, 11, and ≥ 12) and health indicators (poor self-rated health, high

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psychological stress, unhappiness, sexual initiation and pregnancy). Covariates were

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individual- (bodyweight, living with families, parent’s education, household wealth and

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presence of parents and siblings) and community-level factors (year of birth, single-sex

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education and level of school, urbanization level of school area, year of survey and regional

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deprivation). Odds ratios (ORs) for each adverse health indicator were examined by each

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AAM group using multivariable regression analyses. For pregnancy, we calculated relative

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risk (RRs) using log-binomial regression model.

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Results: Age at menarche was < 12 in 42% of our study population. Nearly half of girls born

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in the early 2000s went through menarche before the age of 12 years, whereas only one-

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third of girls born in the early 1990s went through menarche before the age of 12 years.

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Girls who experienced menarche at the age of ≤ 10 or = 11 years were more likely to show

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self-rated poor health (OR = 1.28, 95% confidence intervals (CI): 1.22-1.34 for AAM ≤ 10, 3

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1.16, 95% CI: 1.12-1.21 for AAM = 11), high stress (1.19, 95% CI: 1.14-1.23 and 1.10, 95% CI:

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1.06-1.14), and sexual initiation (2.21, 95% CI:2.05-2.38 and 1.32, 95% CI: 1.23-1.41)

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compared to those with AAM ≥ 12 when we adjust for all covariates. AAM ≤ 10 years was

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associated with consistently higher odds for poor health than AAM ≥ 12 years. The ORs of

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sexual initiation increased with earlier AAM. Risk of pregnancy were similar across AAM

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groups when we control for individual- and community-level covariates.

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Conclusion: Early menarche defined by <12 years can be still a useful indicator in adolescent

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health interventions to identify high risk group in the setting of declining AAM.

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Keywords: Menarche, Adolescent, Female, Health, Stress, Pregnancy

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Word count: 2,957

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Introduction

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Menarche denotes an initiation of the menstrual cycle and usually occurs between the age

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of 12 and 14 years on average.1 Declining age at menarche (AAM) over the last century has

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been reported in many countries.2-7 Although the variation in the timing of menarche has

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known to be largely attributed to genetic factors, these secular trends reflect the role of

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environments in the initiation of menstruation.

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Earlier AAM has been reported to be associated with worse health indicators, such

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as a higher depressive symptom and earlier sexual debut in adolescent girls.8-10 Age limits

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for the definition of early menarche are not definitive. Historically, a number of studies

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regarded an AAM <12-14 years to be early menarche for diverse ethnic groups and

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generations.11-14 Given the global trend of decreasing age at menarche, proportion of girls

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who experience menarche by this definition has increased by 25-33% in recent generations

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of several countries and ethnic groups.15-18 Korean women have shown a fast rate of decline

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in menarche, which is different than noted in other countries.19 Despite the proportion of

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girls who experience menarche before age of 12 years is globally growing, knowledge of

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adverse adolescent health outcomes related with this change is limited. Accordingly, we

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explored compositional change of AAM and association between AAM and adverse

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adolescent health indicators leveraging a national population-based sample of adolescent

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girls born in the 1990s and 2000s.

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Materials and Methods 5

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Data

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We used data of adolescent girls who participated in the 3rd - 11th Korea Youth Risk

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Behaviour Web-based Survey (KYRBS) conducted by the Ministry of Education, Ministry of

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Health and Welfare, Korea Centres for Disease Control and Prevention (KCDC) from 2007 to

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2015. The survey method and data profiles were described previously.20 To be brief, the

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KYRBS is an ongoing national cross-sectional survey that assesses health and health-related

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behaviours of students between the 1st year of middle school (7th grade) and the 3rd year of

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high school (12th grade). A multi-stage cluster sampling is used to obtain a nationally

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representative sample of Korean students. Annually, approximately 70,000 girls and boys,

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corresponding to 2% of the total population of middle- and high-school students, are

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enrolled in the survey.20 All the information used was obtained from the anonymous self-

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administered web-based survey. According to a previous study measuring reliability, the

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percentage agreement was between 77.6% and 100.0% for all indicators.21 After excluding

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those with a missing or implausible AAM (n = 166, <0.1%), the final analysis included

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319,437 participants who aged 12-18 years. This study was exempted from approval by the

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Institutional Review Board of Gangnam CHA hospital (GCI-19-21).

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Exposures

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The primary exposure in this study was AAM in years obtained from the self-administered

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questionnaire (“When did you experience your first menstruation?”). We retrieved timing of 6

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menarche from the response to the question which were scaled from 1 (have not yet

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experienced) to 14 (Grade 12). Then we calculated AAM based on the information of birth

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year and reported grade. To estimate the AAM-specific risk for each health indicator, we

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classified the AAM as 3 groups: ≤10 years, 11 years, and ≥12 years, including

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premenarcheal girls.

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Outcomes

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Four health-related outcomes of interest (Self-rated health, psychological stress,

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unhappiness and adolescent pregnancy) were selected based on recommended indicators to

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measure adolescent health, social development and well-being.22 Sexual initiation in

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adolescence was additionally selected because it is associated with less optimal use of

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contraception and higher risk of sexually transmitted disease exposure.23, 24 Self-rated health

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was measured by single item: “Compared with your friends, how healthy do think you are?”

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The response options were (1) very healthy, (2) relatively healthy, (3) so-so, (4) relatively

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unhealthy, or (5) very unhealthy. These measures in Korean adolescents found to be valid in

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terms of consistent relationship with subjective SES or economic hardship.25 The

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questionnaire for assessing stress was “how often do you feel stress?” and the answers

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included (1) feel very much, (2) feel a lot, (3) feel a little, (4) not feel much, and (5) not feel

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at all. We measured unhappiness using the question: “How much usually do you think you

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are happy?”. The options were (1) Very much, (2) Fairly, (3) A little, (4) Unhappy, and (5) Very 7

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unhappy. For sexual and reproductive history, “Have you ever had sexual intercourse?” was

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used for assessing sexual experience and “Have you ever been pregnant (among those who

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ever had sexual intercourse)?” was used to identify adolescent pregnancy cases. These

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questionnaires showed high test-retest reliability previously.26 For analytical convenience,

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continuous outcome indicators were dichotomized as self-rated poor health (>3 on a 5-point

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scale), high psychological stress (<2- on the 5-point scale) and unhappiness (> 3 on the 5-

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point scale) considering the distribution of responses to each questionnaire. History of

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pregnancy was analysed only among those reported sexual initiation.

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Covariates

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We included the following covariates potentially associated with either AAM or adolescent

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health indicators based on previous evidence: participant’s body mass index (BMI), living

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with families, parent’s education, household wealth, living with family, presence of older or

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younger siblings, year of birth (categorized as 1989-1993, 1994-1998, and 1999-2003),

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single-sex education, level of school (middle, general high, special purpose high),

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urbanization level of school area (rural, town, and city), and regional deprivation index. All

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the information on individual and familial environment was retrieved from self-administered

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questionnaires. Self-reported height and weight from the KYRBS was previously validated.27

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The level of obesity was categorized into four categories based on the classification system

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proposed by World Health Organization for Asians:28 low body weight (body mass index (BMI)

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<18.5 kg/m2), normal body weight (BMI 18.5-23.0 kg/m2), overweight (BMI 23-25 kg/m2),

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and obese (BMI ≥25 kg/m2). Our explanatory models included individual socioeconomic

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status and community environment based on previous findings indicating associations with

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both of AAM and poor health status in adolescence.29-33 We used the parents’ level of

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education (middle school and lower, high school, college or higher) and self-rated household

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wealth as proxy for individual socioeconomic status at the time of menarche. Household

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wealth was assessed by a self-rated 5-scale measure and included as a binary variable

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(higher than middle and lower) in the model. Urbanization level of the school address was

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assigned by the KCDC based on administrative level to three categories (county, large town,

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and city). We used regional deprivation index of school area to measure community

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environment.34

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Statistical analysis

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We conceptualized the relationships among study variables and identify potential

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confounders (Supplemental Fig. 1). We removed the variables which showed a coefficient

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value from Spearman correlation tests larger than 0.9 from the regression models to avoid

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possible redundancy (Supplemental Fig. 2). We calculated the weighted prevalence of each

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of the covariates considering sampling design. Secular trend of compositional change in

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AAM was tested using Cochran-Mantel-Haenszel Statistics. Characteristics of the AAM

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groups were compared for covariates and health indicators. Odds ratios (ORs) for each 9

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adverse health indicator except adolescent pregnancy were examined by each AAM group

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using multivariable regression analyses. For pregnancy, we calculated relative risk (RRs) using

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log-binomial regression model as its relatively high prevalence (>10%) among those with

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sexual experience. We applied three models to address the confounding effect by individual

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and community (school) level factors in the association between AAM and health outcomes.

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In Model 1, we added body mass index (BMI), living with families, parent’s education,

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household wealth, absent parents, older/younger siblings, facility residence, academic

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achievement to empty model (Model 0). In Model 2, we further included year of birth

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(1989-1993, 1994-1998, and 1999-2003), type (girls only or not) and level of school (middle,

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general high, special purpose high), urbanization level of school area, year of survey and

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regional deprivation index to Model 1. Statistical analysis was performed using

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SURVEYLOGISTIC command in SAS Version 9.4 (SAS Institute, Cary, NC, USA).

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Results

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General characteristics of each AAM group

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Of all study participants, AAM≤10, 11 and ≥12 years were 16.5%, 25.5%, and 58.0%. Girls

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with AAM ≤10 years were more likely to be younger, go to the non-girls middle school,

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having parents with higher education, and living in less deprived area compared to their

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counterparts (Table 1). The prevalence of poor health, high psychological stress, 10

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unhappiness, sexual initiation and pregnancy were highest in girls with AAM ≤10 years.

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Risks of poor health, high stress and sexual initiation were lowest when AAM is ≥12 years.

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Compositional change of AAM

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The average participant’s age was 15.0 years (median: 14.5 years). Weighted mean AAM was

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11.9 years (median: 11.3 years). In the total population, 42% experienced menarche before

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age of <12 years. The proportion of girls with an AAM <12 years grew from 32.8% of girls

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born in 1989-1993 to 55.4% of those born in 1999-2003. The percentage of girls who

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experienced menarche at ≤ 10 years increased from 11.1% to 24.4% for the same time

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period (P for trend < 0.001). In those born in 1999-2003, the proportion of AAM <12 years

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(sum of AAM ≤10, and 11 years) exceeded that of AAM ≥ 12 (Fig. 1).

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AAM and adolescent health indicators

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Girls with AAM of 10, and 11 years were more likely to show poor self-rated health, high

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psychological stress and sexual initiation compared to AAM ≥ 12 years (Table 2). The

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associations were robust to different model specifications. AAM ≤ 10 or = 11 years was

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associated with higher odds of poor health, high stress, and sexual initiation in the models 11

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controlling for individual- and community-level covariates (full model, Model 2). For

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unhappiness, only AAM ≤ 10 years group showed consistently higher ORs than AAM ≥ 12

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years in all models. The ORs of sexual initiation increased with earlier AAM. For example, OR

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for sexual initiation was 1.32 (95% CI: 1.23, 1.41) for AAM =11 years and 2.21 (2.05, 2.38) for

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AAM ≤ 10 years when we control for all individual- and community-level variables. Among

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those who experienced sexual initiation, RRs of pregnancy were lower in AAM = 10 or 11

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years than those in the reference group when we use empty although it did not reach

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statistical significance (Model 0). This negative association disappears when we control for

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individual- and community-level covariates (Model 1 & 2). There was no significant

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interaction by three birth cohorts in the association between AAM and all five health

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indicators (data not shown).

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Discussion

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Nearly half of girls born in the early 2000s went through menarche before the age of 12

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years, whereas only one-third of girls born in the early 1990s went through menarche before

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the age of 12 years. Despite the proportion of girls with AAM <12 years reaches record high,

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the odds of poor self-rated health, high psychological stress and sexual initiation were still

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higher in girls who had menarche at age of ≤ 10 or 11 years compared to those with AAM

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≥12 years. It was not evident that adolescent pregnancy risk is higher in these earlier

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menarche girls. The ORs for poor self-rated health, high psychological stress and pregnancy

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were robust across model specification reflecting that possible confounding effect by

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individual- and community-level covariates were limited. This study confirms the predictive

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value of conventional early menarche definition (AAM <12 years) for the worse health and

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well-being status of adolescent girls in the setting of almost half experience AAM before 12

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years.

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This high proportion of girls with menarche before 12 years (overall 42%) in Korea is

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pronounced compared to those of other countries: When defining early menarche as < 12

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years, prevalence of early menarche was 18-25% in the US35, 36, 14.6% in Canada37, 21,7% in

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the UK38, 25.2-27.4% in urban China39, and 6-14% in Indonesia40. This high rate of early

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menarche is assumed to mainly origin from higher availability of food and subsequent

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overweight in early childhood in recent generations.19 In addition, earlier maturation has

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been reported when there is a change in the traditional family structure, social change. The

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rapid social transition across the country would have contributed to earlier menarche in

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Korean girls.41

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There are several possible explanations for this consistent association between

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menarche before age of <12 years and worse health indicators even when a half of girls are

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affected. Higher and fluctuating levels of reproductive hormones occurring at the time of

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menarche may cause more physical and psychological hardship.42 A discrepancy between

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conceived biological, cognitive maturation, and developmental state may induce

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psychological distress and a poor sense of well-being.42, 43 Even feelings of being ‘different’ 13

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to one's peers have been weakened, as AAM decrease in general, our study reveals the

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pronounced risk of poor health and stress did not disappear. This would reveal being ≥12

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years before menarche is important to achieve optimal level of adolescent health. A finding

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of Mendelian randomization study supports the causal effect of AAM itself on depressive

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symptoms in adolescent girls.42 Given that earlier menarche is a well-known risk factor for

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health problems such as obesity, diabetes, cardiovascular diseases, and breast cancer in

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adulthood,44-47 identification of early menarche would have more implication during the

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lifespan of women.

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Our findings are consistent with previous studies reporting worse adolescent health

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in girls who experienced earlier menarche. The definition of early menarche and associated

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health indicators, however, differed across studies. In terms of depressive symptoms during

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adolescence, AAM < 11.6 14 or ≤10 years48 had higher risk compared to their counterparts.

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This finding was not replicated in other study which defines early menarche as ≤12.0

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years.49 Risk of substance use and sexual initiation increased when AAM < 11 years.50 For

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metabolic syndrome, having menarche at age of ≤12.25 years showed a higher sensitivity

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and negative predictive value in overweight/obese girls.51 An AAM ≤11 years was associated

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with a higher risk for premature menopause.11 In a Norwegian study, AAM <13 years was

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associated with a shorter duration of reproductive years.52 Our study revealed odds of

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adolescent pregnancy were not different across AAM groups among sexually active girls. This 14

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variation in cut-off point may be attributed to inconsistency in health indicator of interest,

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difference in the population characteristics, methodology of exposure measures and residual

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confounding effects by family and community-level factors.53 The mixture of difference

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population and community environment would make the association more complicated. For

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example, higher income was associated with higher early menarche in Northern Ghana 54

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but lower risk of early menarche in Canada.37 By considering multiple levels of confounding

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factors, our analysis may provide fundamental insight into the association between AAM and

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adolescent health. Given the consistently higher risk of worse health indicators in AAM ≤10

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and 11 years groups, the conventional definition of early menarche (<12 years) would have

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predictive value regardless of distribution of AAM in the population.

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The strength of this study is a large size, population-based data with diverse risk

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profiles and demonstration of clear decline in AAM in the population during the last decade.

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The prevalence of AAM < 12 years in our study is higher compared to those in the previous

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reports. The model of this study considered individual, familial, school and community

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factors which can potentially confound the association between AAM and adolescent health

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indicators. As most studies that rely on retrospectively reported age at menarche, our study

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may have some degree of recall bias. Survey data was collected at a time close to their

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menarche, therefore, relatively short interval would have minimized the possibility of recall

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bias.53, 55

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Our study has several limitations. First, as a cross-sectional study, we could not

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assess the onset of health-related outcomes. A future prospective study focusing on the

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start of these health-related outcomes will confirm our findings. In assessing health-relate

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outcomes, we did not use multi-dimensional measures which would have provided more

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information. While shorter instruments are more limited than longer tools, they can have

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benefits in terms of ease of interpretation.56 For surveys of adolescent population, a single

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item about their self-rated health was observed to be reliable.57 As an explorative study, our

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findings would provide evidence for the value of early menarche in predicting the general

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health status of an adolescent girl. Disadvantageous early life environments might have

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confounded the association between early menarche and poorer health-related indicators.

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Many of the risk factors for earlier menarche, such as obesity and lower SES, are known to

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be also associated with self-rated poor health and stress in adolescence.30 Specifically,

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stressful environment in early childhood such as father absence and family dysfunction have

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been proposed to account for earlier maturation.33, 58 However, consistent finding of the

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association between various health indicators and earlier menarche in ours and previous

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studies have suggested early menarche itself can be predictive for worse health indicators.

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Second, given that there is accelerated weight gain after menarche, BMI could potentially be

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on the causal pathway between early menarche and adverse health outcomes. However, as

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overweight or obesity has its origin in early childhood and it can also cause earlier puberty,

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BMI at the time of survey could still be regarded as one of the confounders.59, 60 Third,

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questionnaires regarding psychological stress and unhappiness were not validated. Although

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there are not validation studies for the two questions, the consistent associations with

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earlier menarche for the two measures with other health outcomes may support their value 16

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as valid health indicators. In conclusion, while almost a half of adolescent girls experience menarche before

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the age of 12 years, poor health, high stress and unhappiness are more common in girls with

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AAM ≤10, and 11 years compared to those with AAM ≥12 years. This study adds an

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evidence indicating that early menarche defined by < 12 years can still be useful in

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identifying high risk group regardless of compositional change in AAM. In recent generations

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where AAM <12 years is not considered ‘early’ in the population, we still need preventive

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strategies and clinical surveillance for those with AAM < 12years to address the higher risks

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of worse health status during adolescence.

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Acknowledgements

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The authors appreciate the Ministry of Education, Ministry of Health and Welfare, Korea

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Centers for Disease Control and Prevention for providing KYRBS data.

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Funding Information

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This work was supported by the National Research Foundation of Korea grant (NRF-

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2016R1D1A1B03933410 and 2018R1D1A1B07048821) which is funded by the Korean

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Government.

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Conflict of interest: None declared.

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Figure 1. Compositional change of % of each AAM across birth cohort by 5-year interval,

495

319,437 female teenagers (12-18 years old) of the Korea Youth Risk Behavior Web-based

496

Survey, 2007-2015

27

Table 1. Demographic characteristics of each age at menarche group, 319,437 female teenagers (12-18 years old), the Korea Youth Risk Behavior Web-based Survey 2007-2015

≤ 10 years

11 years

≥ 12 years

(N = 53,047)

(N = 81,294)

(N = 185,096)

Weighted frequency

2,641,526

4,089,269

9,301,441

Age (years), median

14.3

14.3

14.8

BMI (kg/m2), mean

20.3

20.2

20.0

Rural

5,970 (6)

8,885 (5.7)

21,956 (6.0)

Town

22,462 (47.6)

34,233 (47.5)

76,349 (46.8)

24,615 (46.4)

38,176 (46.8)

86,791 (47.2)

17,556 (32.4)

29,646 (36.0)

69,763 (37.3)

Middle

32,651 (60.8)

42,847 (52.3)

84,584 (45.8)

General high

15,345 (30.3)

29,946 (38.5)

75,795 (42.6)

5,013 (8.9)

8,454 (9.2)

24,611 (11.6)

2,643 (5.4)

3,874 (4.8)

10,051 (5.4)

High school

17,486 (40.5)

29,033 (41.2)

68,458 (42.1)

College or higher

21,549 (54.1)

34,776 (54.0)

76,819 (52.5)

2,717 (5.8)

4,224 (5.3)

10,451(5.6)

High school

22,751 (52.3)

38,121 (54.1)

90,129 (56.0)

College or higher

17,130 (42.0)

27,063 (40.6)

57,563 (38.3)

Living with family

50,887(96.4)

78,908 (97.7)

178,948 (97.5)

Older

27,319(51.1)

41,509 (50.6)

96,709 (51.8)

Younger

26,528(49.8)

42,783 (52.3)

102,072 (54.8)

18,532 (22.1)

43,225 (22.4)

Age at menarche Variables

Level of urbanization, frequency (%)

City a

Type of school , frequency (%) All-girls school Level of school, frequency (%)

Special purpose high b

Parental education , frequency (%) Father Middle school or lower

Mother Middle school or lower

Presence of siblings, frequency (%)

Self-rated household wealth, frequency (%) Lower than middle

12,569(23.1)

Regional deprivation index, median

-0.07

-0.08

Health outcomes, frequency (%) Poor health

5,006 (9.5)

7,110 (8.8)

15,630 (8.4)

High stress

8,176 (15.5)

11,727 (14.5)

25,886 (13.7)

Unhappiness

1,277 (2.4)

1,427 (1.8)

3,403 (1.8)

Sexual initiation

2,730(5.3)

2,216 (2.7)

4,651 (2.4)

Pregnancyc

755(33.3)

556 (24.9)

1,322 (29.9)

Percentages in parentheses were calculated using weighted frequencies. a

Type of school was missing in 191 participants. bInformation of education was missing in

54,748 (for father) and 49,288 (for mother) girls. cPercentages were calculated only in girls who reported sexual initiation.

Table 2. Risk estimates and confidence intervals (CI) for five health indicators in adolescent girls by age at menarche groups (n= 319,437), the Korea Youth Risk Behavior Web-based Survey 2007-2015.

Model 0

Model 1

Model 2

(No covariates)

(Model 0 + individual covariates)

(Model 1 + cohort, school, community covariates)

≤ 10 years

1.13 (1.08, 1.18)

1.12 (1.07, 1.17)

1.28 (1.22, 1.34)

11 years

1.05 (1.01, 1.10)

1.08 (1.04, 1.13)

1.16 (1.12, 1.21)

≥ 12 years

1.00 (Reference)

1.00 (Reference)

1.00 (Reference)

Age at menarche and health outcomes Poor self-rated health

High psychological stress ≤ 10 years

1.12 (1.08, 1.17)

1.11 (1.07, 1.15)

1.19 (1.14, 1.23)

11 years

1.07 (1.03, 1.10)

1.06 (1.02, 1.09)

1.10 (1.06, 1.14)

≥ 12 years

1.00 (Reference)

1.00 (Reference)

1.00 (Reference)

≤ 10 years

1.19 (1.08, 1.32)

1.18 (1.06, 1.30)

1.22 (1.10, 1.35)

11 years

1.03 (0.94, 1.12)

1.05 (0.96, 1.14)

1.08 (0.99, 1.18)

≥ 12 years

1.00 (Reference)

1.00 (Reference)

1.00 (Reference)

≤ 10 years

1.83 (1.70, 1.97)

1.84 (1.71, 1.98)

2.21 (2.05, 2.38)

11 years

1.14 (1.07, 1.23)

1.18 (1.10, 1.27)

1.32 (1.23, 1.41)

1.00 (Reference)

1.00 (Reference)

1.00 (Reference)

≤ 10 years

0.88 (0.76, 1.01)

1.01 (0.77, 1.32)

1.02 (0.74, 1.39)

11 years

0.93 (0.79, 1.11)

1.10 (0.80, 1.52)

1.08 (0.75, 1.57)

Unhappiness

Sexual initiation

≥ 12 years Pregnancy

a

1.00 (Reference) 1.00 (Reference) 1.00 (Reference) ≥ 12 years For pregnancy, adjusted risk ratio (RR) was calculated. In Model 1, ORs or RRs were adjusted for body mass index (BMI), living with families, parent’s education, household wealth, living with family, presence of older or younger siblings. In Model 2, we added year of birth, single-sex education, level of school (middle, general high, special purpose high), urbanization level of school area (rural, town, and city), year of survey, and regional deprivation index to Model 1.

a