Women aged 35 years and older had a higher risk for giving birth before 34 weeks of gestation but only women aged 35-39 years had a higher risk for stillbirth, according to research published online in Obstetrics & Gynecology.

Line Elmerdahl Frederiksen of Aarhus (Denmark) University and her colleagues compared outcomes for women aged 35-39 years and women aged 40 years or older with outcomes for women aged 20-34 years.

The researchers analyzed 369,516 singleton pregnancies in women who had a first-trimester screening between 11 and 14 weeks of gestation at a public hospital in Denmark between January 2008 and December 2014.

“In this study, we were able to include detected chromosomal abnormalities and congenital malformations from an early gestational age, which in other studies would not have been available for assessment,” the authors wrote. The researchers also included miscarriage, birth before 34 weeks of gestation, and stillbirth outcomes in their analysis.

The researchers reported an increased risk for giving birth before 34 weeks of gestation for women aged 35-39 years (odds ratio, 1.25) and for women aged 40 years or older (OR, 1.66). The authors wrote that this finding was contradictory to previously reported data and noted that obstetric complications, which are known to increase with age, could explain their finding.

Women aged 35-39 years had a higher risk for stillbirth, compared with women aged 20-34 years (OR, 1.43), according to the study. However, the researchers did not observe a similar risk for women aged 40 years or older. “This counterintuitive finding could possibly occur because induction of labor for comorbidities is more likely in women older than 40 years,” wrote Ms. Frederiksen and her coauthors.

The researchers reported increased risk for both miscarriage and chromosomal abnormalities for women aged 35-39 years and women aged 40 years or older, adding to the previously published risk association for older women. Risk for congenital malformations did not appear to differ between maternal age groups, which was also consistent with previous data.

In a composite risk analysis, researchers found an increased risk for an adverse pregnancy outcome for women aged 35-39 years (OR, 1.29) and women aged 40 years or older (OR, 2.02).

The authors reported no potential conflicts of interest.

SOURCE: Frederiksen L et al. Obstet Gynecol. 2018 Feb 5. doi: 10.1097/AOG.0000000000002504.