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Original Investigation
July 2018

Association of Temporal Changes in Gestational Age With Perinatal Mortality in the United States, 2007-2015

Author Affiliations
  • 1Department of Obstetrics and Gynecology, College of Physicians and Surgeons, Columbia University, New York, New York
  • 2Department of Health Policy and Management, Joseph L. Mailman School of Public Health, Columbia University, New York, New York
  • 3Department of Obstetrics and Gynecology, New York University–Winthrop University Hospital, Mineola
JAMA Pediatr. 2018;172(7):627-634. doi:10.1001/jamapediatrics.2018.0249
Key Points

Question  Are changes in the gestational age distribution associated with changes in perinatal mortality?

Findings  In this cohort study of 34 236 577 US singleton births, births at all gestational ages except 39 to 40 weeks decreased from 2007 to 2015; an overall decrease in perinatal mortality rates was attributable to changes in gestational age distribution rather than gestational age–specific mortality. Although the proportion of births at gestational ages 34 to 36, 37 to 38, and 42 to 44 weeks decreased, perinatal mortality at these gestational ages increased.

Meanings  Increased perinatal mortality in some gestational age groups may be associated with lower-risk pregnancies in which neonates are delivered preferentially at 39 to 40 weeks, leaving fetuses at higher risk for mortality at other gestational ages.

Abstract

Importance  Whether the changing gestational age distribution in the United States since 2005 has affected perinatal mortality remains unknown.

Objective  To examine changes in gestational age distribution and gestational age–specific perinatal mortality.

Design, Setting, and Participants  This retrospective cohort study examined trends in US perinatal mortality by linking live birth and infant death data among more than 35 million singleton births from January 1, 2007, through December 31, 2015.

Exposures  Year of birth and changes in gestational age distribution.

Main Outcomes and Measures  Changes in the proportion of births at gestational ages 20 to 27, 28 to 31, 32 to 33, 34 to 36, 37 to 38, 39 to 40, 41, and 42 to 44 weeks; changes in perinatal mortality (stillbirth at ≥20 weeks, and neonatal deaths at <28 days) rates; and contribution of gestational age changes to perinatal mortality. Trends were estimated from log-linear regression models adjusted for confounders.

Results  Among the 34 236 577 singleton live births during the study period, the proportion of births at all gestational ages declined, except at 39 to 40 weeks, which increased (54.5% in 2007 to 60.2% in 2015). Overall perinatal mortality declined from 9.0 to 8.6 per 1000 births (P < .001). Stillbirths declined from 5.7 to 5.6 per 1000 births (P < .001), and neonatal mortality declined from 3.3 to 3.0 per 1000 births (P < .001). Although the proportion of births at gestational ages 34 to 36, 37 to 38, and 42 to 44 weeks declined, perinatal mortality rates at these gestational ages showed annual adjusted relative increases of 1.0% (95% CI, 0.6%-1.4%), 2.3% (95% CI, 1.9%-2.8%), and 4.2% (95% CI, 1.5%-7.0%), respectively. Neonatal mortality rates at gestational ages 34 to 36 and 37 to 38 weeks showed a relative adjusted annual increase of 0.9% (95% CI, 0.2%-1.6%) and 3.1% (95% CI, 2.1%-4.1%), respectively. Although the proportion of births at gestational age 39 to 40 weeks increased, perinatal mortality showed an annual relative adjusted decline of −1.3% (95% CI, −1.8% to −0.9%). The decline in neonatal mortality rate was largely attributable to changes in the gestational age distribution than to gestational age–specific mortality.

Conclusions and Relevance  Although the proportion of births at gestational age 39 to 40 weeks increased, perinatal mortality at this gestational age declined. This finding may be owing to pregnancies delivered at 39 to 40 weeks that previously would have been unnecessarily delivered earlier, leaving fetuses at higher risk for mortality at other gestational ages.

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