Women ages 35 and older give birth to more babies with Down syndrome but fewer with physical defects, new research shows.

Science has long known that women of “advanced maternal age” experience greater risk of bearing a child with a chromosomal abnormality, such as Down syndrome. However, not much has been written on the risk of congenital defects, which may affect the heart, brain, kidney, bones, gastrointestinal tract, and other aspects of the body’s basic composition.

In a new study, investigators at Washington University in St. Louis analyzed obstetric and ultrasound data from more than 76,000 women receiving second trimester ultrasounds as part of their routine neonatal care. Aside from considering the mother’s age, they also looked at the severity of such malformations, categorizing them by organ system, including heart, brain, and kidney.

Overall, older women were 40 percent less likely to give birth to a child with a physical abnormality. Although they gave birth to just as many babies with congenital heart defects, the rate of abnormalities affecting other organ systems was much lower, physician-researcher Katherine R. Goetzinger said in a statement. "As more women are choosing to delay childbearing, they are faced with many increased pregnancy risks," she said. "Findings from this study may provide some reassurance for these women regarding the likelihood of having an anatomically normal child."

Goetzinger, an assistant professor of maternal-fetal medicine, speculated about why older mothers might experience a lower rate of physical birth defects. Perhaps a woman’s body operates in later maternal age under a sense of urgency, rejecting anatomically abnormal fetuses in a “survival of the fittest” effect — with no time for the weak. A similar study last year by Chinese investigators found that older mothers face a lower risk of birthing a child with a congenital heart defect, but higher risk for cleft palate. However, older mothers were more likely to have given birth already and thus less likely of birthing a child with equinovarus, or club foot. The investigators planned to present their findings Thursday in New Orleans to the Society for Maternal-Fetal Medicine’s annual meeting.