Risk of Stillbirth May Be Higher in Prolonged Pregnancies

Risk of stillbirth at term increases markedly for low-risk pregnancies continuing beyond 40 weeks, according to the results of a systematic review and meta-analysis.

Stillbirth risk steadily increased from 0.11 per 1,000 pregnancies at 37 weeks (95% CI 0.07-0.15) to 3.18 per 1,000 at 42 weeks (95% CI 1.84-4.35), reported Shakila Thangaratinam, MBBS, of the Queens Mary University of London, and colleagues.

Patients were 64% more likely (95% CI 1.51-1.77, P<0.001) to have a stillbirth when continuing their pregnancy to 41 weeks -- as currently recommended -- compared to patients who delivered at 40 weeks, researchers wrote in PLOS Medicine.

In black mothers, the risk of stillbirth was up to double that of white mothers at all gestational ages, researchers also noted.

“We were surprised to see how much poorer pregnancy outcomes were for black women,” Thangaratinam said in a statement. “Healthcare professionals need to take these added risks into account when developing care plans for these women.”

In the U.S., approximately 24,000 stillbirths occur per year, according to the CDC. Prolonged pregnancy is a known risk factor for stillbirth, the authors said, and many women are regularly offered early labor induction at 41 weeks to avoid this outcome.

But 1 in 3 stillbirths still occur before 41 weeks gestation, researchers wrote. While Thangaratinam said that current guidelines recommend labor induction at 41 weeks, they do not quantify with specific risk estimates.

“Hence we decided to systematically search for available evidence in this area, to calculate the week by week increase in risk of stillbirth, so that we can provide risk estimates of stillbirth for mothers and healthcare professionals,” she told MedPage Today in an email.

Researchers included 13 studies, the majority of which were from the U.S. and U.K., in their meta-analysis. These studies analyzed over 15 million pregnancies that occurred from 1990 to 2015, reporting almost 18,000 stillbirths and around 2,400 neonatal deaths. Twelve studies reported weekly rates of stillbirth only, 1 provided rates of neonatal death only and 4 provided rates for both. Four of the studies compared the rates of stillbirth between Black and White women, and two of the studies compared rates for Asian and White women.

All studies included clearly defined low-risk pregnancy, excluded pregnancy with congenital fetal malformations and did not include women with multiple pregnancies, researchers wrote.

While researchers found a higher risk of stillbirth in prolonged pregnancies, risk of neonatal death was unchanged for births from 38 and 41 weeks of gestation. At 42 weeks, however, the risk increased 87% compared to 41 weeks (95% CI 1.07-2.86, P = 0.012).

In a sub-group analysis that compared Asian and White women, there was no statistically significant difference in the odds of stillbirth at any gestational age, other than a lower risk of stillbirth at 42 weeks in Asian mothers (RR 0.49, 95% CI 0.29-0.83, P = 0.008).

Eva Pressman, MD, of the University of Rochester Medical Center in Rochester, N.Y., who was not involved with this research, told MedPage Today in an email that this analysis is much larger than previous studies, and reflects the current knowledge about risks of stillbirth in prolonged pregnancies.

“This data supports informed decision making between patients and providers regarding elective deliveries between 40 and 41 weeks,” she said, but noted that understanding these risks is dependent on accurate pregnancy dating.

Thangaratinam also said that this research will help healthcare professionals provide expecting mothers with enough information to understand the magnitude of stillbirth risk when continuing their pregnancy.

“Our findings are aimed to empower women to make informed decisions on timing of delivery,” she said.

Limitations of the study include possible loss of data due to the exclusion of studies that did not track stillbirth and neonatal death in week-by-week intervals. The study is also limited by different definitions of low-risk pregnancy and possible confounders due to the retrospective nature of the research.