BACKGROUND AND PURPOSE:

• Ananth et al. (JAMA Pediatrics, 2018) examined the shifting US gestational age distributions in an effort to determine if there has been any effect on perinatal mortality rates.

METHODS:

• Retrospective cohort study
  • Live birth and infant death data were linked (2007 – 2015)
  • >35 million singleton births
• The study compared the year of birth and changes in gestational age distribution

Outcomes

• Change in proportion of births at 8 gestational age ranges
  • 20 to 27 | 28 to 31 | 32 to 33 | 34 to 36 | 37 to 38 | 39 to 40 | 41 | 42 to 44 weeks
• Changes in perinatal mortality rates
  • Stillbirths at ≥20 weeks
  • Neonatal deaths at <28 days
• Contribution of gestational age distribution changes to perinatal mortality

RESULTS:

• 34,236,577 live births during the study period
• Proportion of births at all gestational ages declined, except at 39 to 40 weeks
  • increased by 54.5% in 2007 and 60.2% 2015
• Perinatal mortality overall declined (9.0 to 8.6 per 1000 births)
  • Stillbirths declined (5.7 to 5.6 per 1000 births)
  • Neonatal mortality declined (3.3 to 3.0 per 1000 births)
• Adjusted annual relative increase in perinatal mortality rates were seen in the following gestational age groups
  • 34 to 36 wks: 1.0% (95% CI, 0.6%-1.4%)
  • 37 to 38 wks: 2.3% (95% CI, 1.9%-2.8%)
  • 42 to 44 wks: 4.2% (95% CI, 1.5%-7.0%)
• Adjusted annual relative decline in perinatal mortality was seen in the following gestational age group
  • 39 and 40 wks: −1.3% (95% CI, −1.8% to −0.9%)
• Adjusted annual relative increase in neonatal mortality rates were seen in the following gestational age groups
  • 34 to 36 wks: 0.9% (95% CI, 0.2%-1.6%)
  • 37 to 38 wks: 3.1% (95% CI, 2.1%-4.1%)
• Decline in neonatal mortality rate was related to change in gestational age distribution and not age–specific mortality

CONCLUSION:

• Decreasing proportion of births at 34-36 weeks and 37-38 weeks may be due to new policies regarding limiting elective deliveries to ≥39 weeks
• Proportion of births at gestational age 39-40 weeks increased, while perinatal mortality at 39-40 weeks declined
• This finding may be the result of 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
• However, the authors note that this study is not designed to determine the cause of why perinatal and neonatal mortality rates increased in the 34-36 and 37-38 week groups and state

“A possible reason for the increased mortality at a gestational age of 37 to 38 weeks could be that physicians may be more likely to defer to 39 weeks for delivery for women at moderately increased risk for adverse perinatal outcomes. With fewer obstetric interventions for early-term pregnancies, the risk may have increased for women approaching 39 weeks’ gestation.”

Learn More – Primary Sources:

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

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