The Changing Epidemiology of Multiple Births in the United States Rebecca B. Russell, MSPH, Joann R. Petrini, MPH, Karla Damus, RN, PhD, Donald R. Mattison, MD, and Richard H. Schwarz, MD OBJECTIVE: To describe changes in the epidemiology of multiple births in the United States from 1980 to 1999 by race, maternal age, and region; and to examine the impact of these changes on birth weight-specific infant mortality rates for singleton and multiple births. METHODS: Retrospective univariate and multivariable analyses were conducted using vital statistics data from the National Center for Health Statistics. RESULTS: Between 1980 and 1999, the overall multiple birth ratio increased 59% (from 19.3 to 30.7 multiple births per 1000 live births, P < .001), with rates among whites increasing more rapidly than among blacks. Women of advanced maternal age, especially those aged 30 –34, 35– 39, and 40 – 44 experienced the greatest increases (62%, 81%, and 110%, respectively). Although all regions of the United States experienced increases in multiple birth ratios between 1991 and 1999, the Northeast had the highest twin (33.9 per 1000 live births) and higher order birth ratios (280.5 per 100,000 live births), even after adjusting for maternal age and race. Between 1989 and 1999, multiple births experienced greater declines in infant mortality than singletons in all birth weight categories. Consequently, very low birth weight and moderately low birth weight infant mortality rates among multiples were lower than among singletons. CONCLUSION: It is important to understand the changing epidemiology of multiple births, especially for women at highest risk (advanced maternal age, white race, Northeast residents). The attribution of infertility management re- quires further study. The differential birth weight-specific infant mortality for singletons and multiples demonstrates the importance of stratifying by plurality when assessing perinatal outcomes. (Obstet Gynecol 2003;101:129 –35. © 2003 by The American College of Obstetricians and Gynecologists.) For years, health care providers were taught that plural births accounted for about 2% of births and 11% of infant deaths. In the 1990s, these proportions changed because of dramatic increases in multiple births. 1,2 Dur- ing this time, there was an upsurge in plural births affecting subgroups of the population, leading to unprec- edented rates of twins, triplets, and other higher order births. By 1999, multiples accounted for 3% of live births (121,628 of 3,959,417) and 14% of infant deaths (4000 of 27,864). 3 These changes have already had significant impact on many perinatal outcomes, particularly gesta- tional age and birth weight, which are strong predictors of infant morbidity and mortality. 2,4 Regardless of man- agement, overall, at least half of all twins and 90% of higher order (triplet and greater) births are low birth weight (LBW) and preterm. 2 This report describes the changing epidemiology of plural births from 1980 to 1999 for the United States providing national statistics by race, maternal age, and region. Plurality-specific in- fant mortality rates are also examined. It is necessary to have an understanding of these background statistics to appreciate the story behind the adjusted rates that are presented. MATERIALS AND METHODS Analyses were based on National Center for Health Statistics natality data from 1980 to 1999 and linked birth/infant death data from 1989 and 1999. The US natality files contain data from all certificates of live births that were submitted through the Vital Statistics Cooperative Program. 5 All states have participated in the program since 1985. For the study years 1980 –1984, data were based on 100% of the participating states and a 50% sample of birth certificates from those states that did not participate. The linked birth infant death data consist of all infant deaths that have been linked to their corresponding birth certificates for 1989 and 1999. 3 A From the Perinatal Data Center, March of Dimes Birth Defects Foundation, White Plains, New York; Albert Einstein College of Medicine, Bronx, New York; National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland; and New York Methodist Hospital and Weill College of Medicine of Cornell University, New York, New York. The authors acknowledge Howard Andrews, PhD, Mailman School of Public Health, Columbia University, and the New York State Psychiatric Institute, for statistical consultation and advice. 129 VOL. 101, NO. 1, JANUARY 2003 0029-7844/03/$30.00 © 2003 by The American College of Obstetricians and Gynecologists. Published by Elsevier Science Inc. PII S0029-7844(02)02316-5