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Increasing trends in plural births in the united states
Increasing Trends in Plural Births in the United States SANDRA E. /E WELL, MA, AND RAY YIP, MD, MPH Objective: To determine the nature and possible reasons for the increasing trend in plural births in the United States during the 1980s. Methods: We performed a descriptive analysis of births in the United States for five racial and ethnic groups from 1980-1989, using the United States vital records natality files. Results: The rates of twin and triplet births rose 19 and lOO%, respectively, during the 1980s. Approximately onefourth of the observed increases can be attributed to rising maternal age. The increases in twin and triplet births occurred mainly among more educated and older white women. Conclusion: The association of high education status with rising rates of plural births, independent of maternal age, suggests that the observed increase is the result of increasing use of fertility-stimulating therapy among a subset of the childbearing population. (Obstet Gynecol 1995;85:229-32)
Plural births, which account for approximately 2% of births each year, often result in low birth weight and greater risk of morbidity and mortality than singleton births.‘,’ Recent studies show that plural births place an important burden on medical care resources and carry much higher social and economic costs than singleton births.3-5 Plural births occur proportionally more often to black women and to older women of all races.6Z7 There were reports *f9 in the 1980s of an increasing trend in the rate of plural births during the same period that great changes were occurring in the demographic composition of childbearing women in the United States.70’1’ We conducted this study to determine whether the upward trend in plural births was the result of shifting age and race or ethnicity of women of childbearing age or was related to factors independent of demographic changes. We used United States birth records from 19861989 to characterize the role of race,
From the Division of Nutrition, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia.
VOL.
85, NO.
2, FEBRUARY
1995
education, and maternal age in the rising incidence of plural births during the decade.
Materials and Methods The birth records used in this study came from the National Center for Health Statistics (NCHS) Natality Records for the years 1980-1989.‘2 Every state sends data on reported births to the NCHS annually and uses a standard format that includes such information as parents’ marital status, age, race, and education, and the infants’ birth weight and plurality. Until 1985, when all areas began submitting 100% of birth records, some states and the District of Columbia sent a 50% sample. This study weights the sampled records to simulate 100% coverage for those areas and earlier years. To derive the infant mortality rates associated with plural status, we used the NCHS vital records Linked Birth/Infant Death Data Sets for the birth cohorts 1983-l 987, r3 the only years during the 1980s for which such records are currently available. For this study, the term “plural birth” refers to any delivery of more than one infant, the term “twin birth” refers to two infants, and the term “triplet birth” refers to three or more infants. Until 1989, when coding standards changed, NCHS natality records recorded the plurality of any delivery of three or more infants as “3.” For this reason, the designation “triplet birth” includes a few births of higher-order multiples. We treated each infant born in a plural delivery as a separate birth and not as a fraction of a multiple birth. Therefore, birth rates as presented reflect the number of births and not the number of pregnancies. The race and ethnic categories used in this analysis were white, black, Asian, American Indian, and Hispanic. The race of the infant was determined from the race and ethnic classification of both parents. Except for the overall trend analyses of maternal age and education, we used only those births for which both parents had the same race. When both parents were of Hispanic
0029-7844/95/$9.50 0029-7844(94)00354-G
229
origin, the births were tabulated as a separate ethnic category, regardless of race. The requirement that both parents belong to the same specified race or ethnic group allowed a clearer delineation of race-specific trends, but resulted in the loss of seven million records, 18% of the IO-year total, and an increasing proportion of records each year. For example, in 1980, 15.6% of records were rejected because either the parents didn’t have the same race or the race of the father was not specified; in 1989, 21.9% of records were rejected for these reasons. The elimination of these records did not result in any significant change in either rates or the trends observed for plural births. The overall trends in maternal age and education between 1980-1989 were calculated using total birth records for each year. The number of these records increased annually and ranged from 3,617,981 in 1980 to 4,045,693 in 1989. Included in the analysis of births by race and ethnic group were 30,715,005 births, of which 23,725,433 were births to white parents, 3,579,907 were births to black parents, 2,984,871 were births to Hispanic parents, 284,628 were births to Asian parents, and 140,166 were births to American Indian parents. Because our findings are based on the entire population of qualifying births each year instead of a sample, no standard errors or other sample statistics are presented in this analysis. Age or race and ethnic categories that had fewer than 25 births were excluded from analysis. This limited the analysis of triplet births for women less than age 20 and for Asian and American Indian women. We used these six maternal age groups: under 20 years old, 20-24,25-29,30-34,35-39, and 40 and over. Because of the small number of triplet births among women older than 34 years, triplet births among women ages 35-39 years and 40 or more years were combined to calculate a rate for ages over 34. We generated age- and race-adjusted rates using the indirect method of standardization and 1980 as the base year. We used simple linear regression to assess the unadjusted and age- and race-adjusted trends in twin and triplet birth rates and in maternal age and education. Significance was defined as P < .05. All quoted P values refer to the significance of the trend between 1980-1989.
Results Between 1980-1989, the average annual rate of twin births in the United States for the five combined race and ethnic categories was 20.6 per 1000 live births, and the rate of triplet births was 5.3 per 10,000 live births. Although twin and triplet births constituted a small percentage of all births, they contributed disproportion-
230
Jewel1
and
Yip
Plurd
Births
35,
04CM
20-24
25-29
Mothers’ Figure 1. Rates of twin American Indian women
30-34
35-39
z-39
Age Group
births to white, black, Hispanic, Asian, by maternal age group, 1980-1989.
and
ately to infant mortality in the United States. From 1983-1987, the infant mortality rate for twins was 52.7 per 1000 live births and for triplets, 138.5 per 1000, compared to an infant mortality rate of 9.7 per 1000 live births for singletons. The incidence of plural births varied considerably among race and ethnic groups. Asian women had the lowest rate of twin births during the decade, 14.3 per 1000 live births, and black women had the highest, 26.7 per 1000 live births. White women had the highest rate of triplet births, 5.9 per 10,000 live births. In general, the proportion of twin births increased with maternal age. For white, black, and Hispanic women, it peaked between the ages of 35-39 years. The rates for Asian and American Indian women followed the same pattern, but continued to increase beyond age 39 (Figure 1). The incidence of triplet births was highest for Hispanic women between the ages of 30-34, and for white and black women at ages over 34. The rate of triplet births to white women rose substantially after the age of 24 (Figure 2). Asian and American Indian women had few triplet births and were excluded from this part of the analysis. Twin-birth rates in the United States grew annually between 1980-1989, from 19.0 to 22.6 per 1000 live births (P < .OOOl), representing an increase of 19%. This increase was primarily attributable to births among white women, who contributed almost 80% of all births in the United States during those years. Twin births to white women rose 23% between 1980-1989, from 18.4 to 22.7 per 1000 live births (P < .OOOl). There was a
10 , 98-
While
765432l-
0’
I 40
20-24
25-29
Mothers’
30-34
34
Age Group
Figure 2. Rates of triplet births to white, by maternal age group, 1980-1989.
black,
and Hispanic
women
positive trend for twin births to black women, who experienced a 9% increase during the decade (P = .Ol). There were less certain trends in twin births to Asian and Hispanic women, and no trend in births to American Indian women. The rate of triplet births escalated even more rapidly than the rate of twin births. Triplet births to all women rose from 3.9 per 10,000 live births in 1980 to 7.8 per 10,000 in 1989, an increase of 100% (P < .OOOl).White women were responsible for most of this trend. Triplet births to white women rose from 4.1 per 10,000 live births in 1980 to 9.2 per 10,000 in 1989, an increase of 124%. There was no significant trend in triplet births to black or Hispanic women. Twin birth rates to white women rose among all age groups during the 198Os,but dramatic increases in twin birth rates began over age 24. Between 1980-1989, twin birth rates to women ages 25-29 increased 17%; to women ages 30-34, 21%; to women ages 35-39, 25%; and to women age 40 and over, 21%. White women at ages over 24 had an accelerating rate of triplet births during the 1980s. For these women, the rate of triplet births at ages 25-29 years grew 87% during the decade; at ages 30-34 years, it increased 150%; and at ages 35 and over, the birth rate of triplets grew 179%. The median age of mothers of all races advanced steadily during the 1980s. In 1980, the median mother’s age was 25.5 years, and in 1989, the median age was 27.3 (P < .OOOl).Because older mothers have naturally occurring higher rates of plural births, this upward shift in maternal age would be expected to contribute to
VOL.
85, NO.
2, FEBRUARY
1995
increasing rates of twin and triplet births. To measure the impact of increasing maternal age, rates were ageadjusted for each race. This adjustment reduced the rising trend for twin and triplet births observed for white women and eliminated the upward shift in twin births to black, Asian, and Hispanic women. The age-adjusted rate of twin births to white women rose 17% (P = .OOOl)between 1980-1989. As a consequence, less than 30% of the increase in the unadjusted rate of twin births to white women (23%) can be attributed to increases in maternal age during the decade. -After adjusting for age, the rate of triplet births to white mothers increased 102%. Changes in age composition, therefore, accounted for only 19% of the increase of triplet birth rates among white women. As the decade went on, the education level of mothers increased. A progressively greater proportion of mothers of all races completed high school (12 years of education) as well as college (16 years of education) in each subsequent year of the decade (P < .OOOl).However, only white women showed a clear and consistent relation between education attainment and plurality, especially triplet births. In 1989, the age-adjusted rate of triplet births was 67% higher for college-educated white women than for white women of the same age with less than a high school education (Table 1).
Discussion This study documented a significant increase of plural births during the 1980s. Because older mothers have higher rates of plural births, the 2-year increase in median maternal age during the same time period accounted for approximately 20-30% of the observed increase in twin and triplet births. We also found that the increase was race-specific, affecting mainly white women. Among white mothers, those with higher levels of education had significantly greater rates of plural births than the same age women with less education. This tendency for higher rates of plural births among
Table
1.
Triplet Births to White Women by Education Attainment and Age Group per 10,000 Live Births, 1989 il2th grade
High school
Some college
20-24 25-29 30 -34 235
2.2 5.0 5.4 *
3.2 6.1 14.9 6.1
3.0 10.6 13.9 14.5
10.5 12.9 18.9 18.3
All ages Age-adjusted
3.7 5.7
6.9 6.9
10.2 7.9
16.2 9.5
AiF group
Graduated college
* Less than 25 births
Jewel1
and
Yip
Phd
Births
231
more-educated women strongly suggests that behavioral factors contributed to the trend. One likely explanation is the preferential use of fertility-stimulating therapy by these women.r4,i5 It is well known that fertility-stimulating therapy increases the risk for plural births.‘6,‘7 Recent estimates from the NCHS National Survey of Family Growth conducted in 1982 and 1988 indicated that the use of ovulation stimulants may have increased by 75% between those years.l’ The marked increase in plural birth rates as well as the increasing use of fertility-stimulating therapy are cause for concern. In the past 30 years, infant mortality declined dramatically for both singleton and plural births.2*9 However, in the 198Os,the small but growing minority of plural births still contributed disproportionately to infant deaths: twins had five times the mortality rate of singletons, and triplets 14 times the singleton rate. A recent analysis by Callahan et al3 confirmed that assisted-reproduction techniques resulted in higher rates of plural births and calculated that the perinatal health care costs associated with a plural birth were four times higher for twins and 11 times higher for triplets than those of a singleton birth. Such information, together with the increasing national trend in plural births documented by this study, points to the need for the development of an effective fertilitystimulating therapy that carries a lower risk for twin and higher order births.
7. Taffel SM. Health and demographic characteristics of twin births: United States, 1988. Vital and health statistics; series 21 no. 34. Hyattsville, Maryland: National Center for Health Statistics, 1992. 8. Allen G. The non-decline in U.S. twin birth rates, 1964-1983. Acta Genet Med Gemellol 1987;36:313-23. 9. Kiely JL, Kleinman JC, Kiely M. Triplets and higher order multiple births, time trends and infant mortality. Am J Dis Child 1992;146: 862-8. 10. Bureau of the Census. General population characteristics, 1980 census of population. 1: Chapter B. Washington, DC: United States Department of Commerce; 1981:pcSO-l-Bl. 11. Bureau of the Census. General population characteristics, 1990 census of population. 1:l. Washington, DC: United States Department of Commerce; 1992:cp-1-l. 12. National Center for Health Statistics. Public use data tape documentation: Detail natality. Hyattsville, Maryland: Public Health Service, 1982-1991. 13. National Center for Health Statistics. Public use data tape documentation: Linked birth/infant death data set. Hyattsville, Maryland: Public Health Service. 1989-1991. 14. Wilcox LS, Mosher WD. Use of infertility services in the United States. Obstet Gyncol 1993;82:122-7. 15. Horn MC, Mosher WD. National Center for Health Statistics. Use of services for family planning and infertility, United States, 1982. Vital and health statistics; series 23 no. 13. Hyattsville, Maryland: Public Health Service. 1987. 16. Steptoe PC, Edwards RG, Walters DE. Observations on 767 clinical pregnancies and 500 births after human in vitro fertilization. Hum Reprod 1986;1:89-94. 17. Zimmerman R, Soor B, Braendle W, Lehmann F, Weise HC, Bettendorf G. Gonadotropin therapy of female infertility. Analysis of results in 416 cases. Gynecol Obstet Invest 1982;14:1-18. 18. National Center for Health Statistics. National survey of family growth, 1982 and 1988. Hyattsville, Maryland: Public Health Service.
References 1. McCarthy BJ, Sachs BP, Layde PM, Burton A, Terry JS, Rochat R. The epidemiology of neonatal death in twins. Am J Obstet Gynecol 1981;141:252-6. 2. Kleinman JC, Fowler MG, Kessel SS. Comparison of infant mortality among twins and singletons: United States 1960 and 1983. Am J of Epidemiol 1991;133:133-43. 3. Callahan TL, Hall JE, Ettner SL, Christiansen CL, Greene MF, Crowley WF. The economic impact of multiple-gestation pregnancies and the contribution of assisted-reproduction techniques to their incidence. N Engl J Med 1994;331:244-9. 4. Collins JA. Reproductive technology-The price of progress. N Engl J Med 1994;331:270-1. 5. Malmstrom PM, Biale R. An agenda for meeting the special needs of multiple birth families. Acta Genet Med Gemellol 1990;39:50714. 6. Metropolitan Life Insurance Company. Multiple births: An upward trend in the United States. Stat Bull Metrop Insur Co 1988;69:10-5.
232
Jewel1
and
Yip
Plural
Births
Address
reprint
requests
to:
Sandra E. Jewell, MA Centers for Disease Control and Prevention 4770 Buford Highway, N.E. Mailstop K26 Atlanta, GA 30341-3724
Received Received Accepted
May 10, 1994. in revised form September September 19, 1994.
Copyright 0 Gynecologists.
1995 by The
American
7, 1994.
College
of Obstetricians
Obstetrics
6 Gynecology
and
Plural births, which account for approximately 2% of births each year, often result in low birth weight and greater risk of morbidity and mortality than singleton births.‘,’ Recent studies show that plural births place an important burden on medical care resources and carry much higher social and economic costs than singleton births.3-5 Plural births occur proportionally more often to black women and to older women of all races.6Z7 There were reports *f9 in the 1980s of an increasing trend in the rate of plural births during the same period that great changes were occurring in the demographic composition of childbearing women in the United States.70’1’ We conducted this study to determine whether the upward trend in plural births was the result of shifting age and race or ethnicity of women of childbearing age or was related to factors independent of demographic changes. We used United States birth records from 19861989 to characterize the role of race,
From the Division of Nutrition, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia.
VOL.
85, NO.
2, FEBRUARY
1995
education, and maternal age in the rising incidence of plural births during the decade.
Materials and Methods The birth records used in this study came from the National Center for Health Statistics (NCHS) Natality Records for the years 1980-1989.‘2 Every state sends data on reported births to the NCHS annually and uses a standard format that includes such information as parents’ marital status, age, race, and education, and the infants’ birth weight and plurality. Until 1985, when all areas began submitting 100% of birth records, some states and the District of Columbia sent a 50% sample. This study weights the sampled records to simulate 100% coverage for those areas and earlier years. To derive the infant mortality rates associated with plural status, we used the NCHS vital records Linked Birth/Infant Death Data Sets for the birth cohorts 1983-l 987, r3 the only years during the 1980s for which such records are currently available. For this study, the term “plural birth” refers to any delivery of more than one infant, the term “twin birth” refers to two infants, and the term “triplet birth” refers to three or more infants. Until 1989, when coding standards changed, NCHS natality records recorded the plurality of any delivery of three or more infants as “3.” For this reason, the designation “triplet birth” includes a few births of higher-order multiples. We treated each infant born in a plural delivery as a separate birth and not as a fraction of a multiple birth. Therefore, birth rates as presented reflect the number of births and not the number of pregnancies. The race and ethnic categories used in this analysis were white, black, Asian, American Indian, and Hispanic. The race of the infant was determined from the race and ethnic classification of both parents. Except for the overall trend analyses of maternal age and education, we used only those births for which both parents had the same race. When both parents were of Hispanic
0029-7844/95/$9.50 0029-7844(94)00354-G
229
origin, the births were tabulated as a separate ethnic category, regardless of race. The requirement that both parents belong to the same specified race or ethnic group allowed a clearer delineation of race-specific trends, but resulted in the loss of seven million records, 18% of the IO-year total, and an increasing proportion of records each year. For example, in 1980, 15.6% of records were rejected because either the parents didn’t have the same race or the race of the father was not specified; in 1989, 21.9% of records were rejected for these reasons. The elimination of these records did not result in any significant change in either rates or the trends observed for plural births. The overall trends in maternal age and education between 1980-1989 were calculated using total birth records for each year. The number of these records increased annually and ranged from 3,617,981 in 1980 to 4,045,693 in 1989. Included in the analysis of births by race and ethnic group were 30,715,005 births, of which 23,725,433 were births to white parents, 3,579,907 were births to black parents, 2,984,871 were births to Hispanic parents, 284,628 were births to Asian parents, and 140,166 were births to American Indian parents. Because our findings are based on the entire population of qualifying births each year instead of a sample, no standard errors or other sample statistics are presented in this analysis. Age or race and ethnic categories that had fewer than 25 births were excluded from analysis. This limited the analysis of triplet births for women less than age 20 and for Asian and American Indian women. We used these six maternal age groups: under 20 years old, 20-24,25-29,30-34,35-39, and 40 and over. Because of the small number of triplet births among women older than 34 years, triplet births among women ages 35-39 years and 40 or more years were combined to calculate a rate for ages over 34. We generated age- and race-adjusted rates using the indirect method of standardization and 1980 as the base year. We used simple linear regression to assess the unadjusted and age- and race-adjusted trends in twin and triplet birth rates and in maternal age and education. Significance was defined as P < .05. All quoted P values refer to the significance of the trend between 1980-1989.
Results Between 1980-1989, the average annual rate of twin births in the United States for the five combined race and ethnic categories was 20.6 per 1000 live births, and the rate of triplet births was 5.3 per 10,000 live births. Although twin and triplet births constituted a small percentage of all births, they contributed disproportion-
230
Jewel1
and
Yip
Plurd
Births
35,
04CM
20-24
25-29
Mothers’ Figure 1. Rates of twin American Indian women
30-34
35-39
z-39
Age Group
births to white, black, Hispanic, Asian, by maternal age group, 1980-1989.
and
ately to infant mortality in the United States. From 1983-1987, the infant mortality rate for twins was 52.7 per 1000 live births and for triplets, 138.5 per 1000, compared to an infant mortality rate of 9.7 per 1000 live births for singletons. The incidence of plural births varied considerably among race and ethnic groups. Asian women had the lowest rate of twin births during the decade, 14.3 per 1000 live births, and black women had the highest, 26.7 per 1000 live births. White women had the highest rate of triplet births, 5.9 per 10,000 live births. In general, the proportion of twin births increased with maternal age. For white, black, and Hispanic women, it peaked between the ages of 35-39 years. The rates for Asian and American Indian women followed the same pattern, but continued to increase beyond age 39 (Figure 1). The incidence of triplet births was highest for Hispanic women between the ages of 30-34, and for white and black women at ages over 34. The rate of triplet births to white women rose substantially after the age of 24 (Figure 2). Asian and American Indian women had few triplet births and were excluded from this part of the analysis. Twin-birth rates in the United States grew annually between 1980-1989, from 19.0 to 22.6 per 1000 live births (P < .OOOl), representing an increase of 19%. This increase was primarily attributable to births among white women, who contributed almost 80% of all births in the United States during those years. Twin births to white women rose 23% between 1980-1989, from 18.4 to 22.7 per 1000 live births (P < .OOOl). There was a
10 , 98-
While
765432l-
0’
I 40
20-24
25-29
Mothers’
30-34
34
Age Group
Figure 2. Rates of triplet births to white, by maternal age group, 1980-1989.
black,
and Hispanic
women
positive trend for twin births to black women, who experienced a 9% increase during the decade (P = .Ol). There were less certain trends in twin births to Asian and Hispanic women, and no trend in births to American Indian women. The rate of triplet births escalated even more rapidly than the rate of twin births. Triplet births to all women rose from 3.9 per 10,000 live births in 1980 to 7.8 per 10,000 in 1989, an increase of 100% (P < .OOOl).White women were responsible for most of this trend. Triplet births to white women rose from 4.1 per 10,000 live births in 1980 to 9.2 per 10,000 in 1989, an increase of 124%. There was no significant trend in triplet births to black or Hispanic women. Twin birth rates to white women rose among all age groups during the 198Os,but dramatic increases in twin birth rates began over age 24. Between 1980-1989, twin birth rates to women ages 25-29 increased 17%; to women ages 30-34, 21%; to women ages 35-39, 25%; and to women age 40 and over, 21%. White women at ages over 24 had an accelerating rate of triplet births during the 1980s. For these women, the rate of triplet births at ages 25-29 years grew 87% during the decade; at ages 30-34 years, it increased 150%; and at ages 35 and over, the birth rate of triplets grew 179%. The median age of mothers of all races advanced steadily during the 1980s. In 1980, the median mother’s age was 25.5 years, and in 1989, the median age was 27.3 (P < .OOOl).Because older mothers have naturally occurring higher rates of plural births, this upward shift in maternal age would be expected to contribute to
VOL.
85, NO.
2, FEBRUARY
1995
increasing rates of twin and triplet births. To measure the impact of increasing maternal age, rates were ageadjusted for each race. This adjustment reduced the rising trend for twin and triplet births observed for white women and eliminated the upward shift in twin births to black, Asian, and Hispanic women. The age-adjusted rate of twin births to white women rose 17% (P = .OOOl)between 1980-1989. As a consequence, less than 30% of the increase in the unadjusted rate of twin births to white women (23%) can be attributed to increases in maternal age during the decade. -After adjusting for age, the rate of triplet births to white mothers increased 102%. Changes in age composition, therefore, accounted for only 19% of the increase of triplet birth rates among white women. As the decade went on, the education level of mothers increased. A progressively greater proportion of mothers of all races completed high school (12 years of education) as well as college (16 years of education) in each subsequent year of the decade (P < .OOOl).However, only white women showed a clear and consistent relation between education attainment and plurality, especially triplet births. In 1989, the age-adjusted rate of triplet births was 67% higher for college-educated white women than for white women of the same age with less than a high school education (Table 1).
Discussion This study documented a significant increase of plural births during the 1980s. Because older mothers have higher rates of plural births, the 2-year increase in median maternal age during the same time period accounted for approximately 20-30% of the observed increase in twin and triplet births. We also found that the increase was race-specific, affecting mainly white women. Among white mothers, those with higher levels of education had significantly greater rates of plural births than the same age women with less education. This tendency for higher rates of plural births among
Table
1.
Triplet Births to White Women by Education Attainment and Age Group per 10,000 Live Births, 1989 il2th grade
High school
Some college
20-24 25-29 30 -34 235
2.2 5.0 5.4 *
3.2 6.1 14.9 6.1
3.0 10.6 13.9 14.5
10.5 12.9 18.9 18.3
All ages Age-adjusted
3.7 5.7
6.9 6.9
10.2 7.9
16.2 9.5
AiF group
Graduated college
* Less than 25 births
Jewel1
and
Yip
Phd
Births
231
more-educated women strongly suggests that behavioral factors contributed to the trend. One likely explanation is the preferential use of fertility-stimulating therapy by these women.r4,i5 It is well known that fertility-stimulating therapy increases the risk for plural births.‘6,‘7 Recent estimates from the NCHS National Survey of Family Growth conducted in 1982 and 1988 indicated that the use of ovulation stimulants may have increased by 75% between those years.l’ The marked increase in plural birth rates as well as the increasing use of fertility-stimulating therapy are cause for concern. In the past 30 years, infant mortality declined dramatically for both singleton and plural births.2*9 However, in the 198Os,the small but growing minority of plural births still contributed disproportionately to infant deaths: twins had five times the mortality rate of singletons, and triplets 14 times the singleton rate. A recent analysis by Callahan et al3 confirmed that assisted-reproduction techniques resulted in higher rates of plural births and calculated that the perinatal health care costs associated with a plural birth were four times higher for twins and 11 times higher for triplets than those of a singleton birth. Such information, together with the increasing national trend in plural births documented by this study, points to the need for the development of an effective fertilitystimulating therapy that carries a lower risk for twin and higher order births.
7. Taffel SM. Health and demographic characteristics of twin births: United States, 1988. Vital and health statistics; series 21 no. 34. Hyattsville, Maryland: National Center for Health Statistics, 1992. 8. Allen G. The non-decline in U.S. twin birth rates, 1964-1983. Acta Genet Med Gemellol 1987;36:313-23. 9. Kiely JL, Kleinman JC, Kiely M. Triplets and higher order multiple births, time trends and infant mortality. Am J Dis Child 1992;146: 862-8. 10. Bureau of the Census. General population characteristics, 1980 census of population. 1: Chapter B. Washington, DC: United States Department of Commerce; 1981:pcSO-l-Bl. 11. Bureau of the Census. General population characteristics, 1990 census of population. 1:l. Washington, DC: United States Department of Commerce; 1992:cp-1-l. 12. National Center for Health Statistics. Public use data tape documentation: Detail natality. Hyattsville, Maryland: Public Health Service, 1982-1991. 13. National Center for Health Statistics. Public use data tape documentation: Linked birth/infant death data set. Hyattsville, Maryland: Public Health Service. 1989-1991. 14. Wilcox LS, Mosher WD. Use of infertility services in the United States. Obstet Gyncol 1993;82:122-7. 15. Horn MC, Mosher WD. National Center for Health Statistics. Use of services for family planning and infertility, United States, 1982. Vital and health statistics; series 23 no. 13. Hyattsville, Maryland: Public Health Service. 1987. 16. Steptoe PC, Edwards RG, Walters DE. Observations on 767 clinical pregnancies and 500 births after human in vitro fertilization. Hum Reprod 1986;1:89-94. 17. Zimmerman R, Soor B, Braendle W, Lehmann F, Weise HC, Bettendorf G. Gonadotropin therapy of female infertility. Analysis of results in 416 cases. Gynecol Obstet Invest 1982;14:1-18. 18. National Center for Health Statistics. National survey of family growth, 1982 and 1988. Hyattsville, Maryland: Public Health Service.
References 1. McCarthy BJ, Sachs BP, Layde PM, Burton A, Terry JS, Rochat R. The epidemiology of neonatal death in twins. Am J Obstet Gynecol 1981;141:252-6. 2. Kleinman JC, Fowler MG, Kessel SS. Comparison of infant mortality among twins and singletons: United States 1960 and 1983. Am J of Epidemiol 1991;133:133-43. 3. Callahan TL, Hall JE, Ettner SL, Christiansen CL, Greene MF, Crowley WF. The economic impact of multiple-gestation pregnancies and the contribution of assisted-reproduction techniques to their incidence. N Engl J Med 1994;331:244-9. 4. Collins JA. Reproductive technology-The price of progress. N Engl J Med 1994;331:270-1. 5. Malmstrom PM, Biale R. An agenda for meeting the special needs of multiple birth families. Acta Genet Med Gemellol 1990;39:50714. 6. Metropolitan Life Insurance Company. Multiple births: An upward trend in the United States. Stat Bull Metrop Insur Co 1988;69:10-5.
232
Jewel1
and
Yip
Plural
Births
Address
reprint
requests
to:
Sandra E. Jewell, MA Centers for Disease Control and Prevention 4770 Buford Highway, N.E. Mailstop K26 Atlanta, GA 30341-3724
Received Received Accepted
May 10, 1994. in revised form September September 19, 1994.
Copyright 0 Gynecologists.
1995 by The
American
7, 1994.
College
of Obstetricians
Obstetrics
6 Gynecology
and