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Perinatal Mortality in First- and Second-Born Twins in the United States

From the Division of Epidemiology, School of Public Health, University of Medicine and Dentistry of New Jersey, Section of Epidemiology and Biostatistics, and Division of Maternal-Fetal Medicine Department of Obstetrics, Gynecology and Reproductive Sciences, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School/Robert Wood Johnson University Hospital, New Brunswick, New Jersey.

Address reprint requests to: Cande V. Ananth, PhD, MPH, Department of Obstetrics, Gynecology and Reproductive Sciences, UMDNJ-Robert Wood Johnson Medical School, 125 Paterson Street, New Brunswick, NJ 08901; e-mail: cande.ananth{at}UMDNJ.EDU.

OBJECTIVE: To evaluate the prevailing mortality paradox that second-born twins are at higher risk of perinatal mortality than first-born twins.

METHODS: We used the 1995–1997 United States "matched multiple birth" data files assembled by the National Center for Health Statistics, for analysis of risk of perinatal mortality in first- and second-born twins (293,788 fetuses). Perinatal mortality was defined to include stillbirths after 20 weeks of gestation and neonatal deaths (deaths within the first 28 days). Gestational age-specific risk of perinatal mortality (per 1,000 total births), stillbirth (per 1,000 total births), and neonatal mortality (per 1,000 livebirths) by order of twin birth were based on the fetuses-at-risk approach. Associations between order of birth and mortality indices were evaluated by fitting multivariable logistic regression models based on the method of generalized estimating equations. These models were adjusted for several potential confounding factors.

RESULTS: Perinatal mortality was 37% higher in second-born (26.1 per 1,000 total births) than in first-born (20.3 per 1,000 total births) twins (adjusted relative risk [RR] 1.37; 95% confidence interval [CI] 1.32, 1.42). The increased risk of perinatal mortality in second-born twins was chiefly driven by a 2.46-fold (95% CI 2.29, 2.63) increase in the number of stillbirths. However, the risk of neonatal mortality was very similar between first- and second-born twins (RR 0.99, 95% CI 0.95, 1.04).

CONCLUSIONS: The increased risk of perinatal death in second-born twins is driven chiefly by increased rates of stillborn second twins. Thus, the increased mortality in second-born over first-born twins probably is an artifact of mortality comparisons.

LEVEL OF EVIDENCE: II-2

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