A new study suggests that taking folic acid in late pregnancy may increase the risk of allergies in offspring affected by intrauterine growth restriction (IUGR). The article is published ahead of print in the American Journal of Physiology—Regulatory, Integrative and Comparative Physiology.

Folic acid, a type of B vitamin, has been shown to prevent defects in the neural tube—the precursor to the central nervous system—in a developing fetus. The neural tube develops in the first month of pregnancy; medical professionals typically recommend women take a folic acid supplement during the first trimester of pregnancy. Continued supplementation, however, may not be needed in the late stages of pregnancy and may actually increase the risk of allergies in offspring.

Previous research has also shown that IUGR—a form of growth restriction in the womb often resulting in lower birth weights—may have a protective effect against childhood allergies. The risk of allergy involved with both of these factors at the same time is less clear.

Australian researchers at the University of Adelaide’s Robinson Research Institute studied lambs born to three groups of sheep:

• mothers with a smaller-than-normal placenta (“restricted”);
• mothers with a smaller placenta that were also given high doses of a supplement that included folic acid in the last month of gestation (“restricted supplement”); and
• mothers with normal placenta and normal diet (“control”).

The research team measured systemic inflammation and tested skin reactions—markers of allergies—to the common allergens dust mites and egg whites in the lambs. The restricted group had higher levels of inflammation, but no difference in skin reaction than the restricted supplement and control groups when exposed to dust mites. However, when tested with egg white protein, the restricted supplement and control groups showed higher rates of allergic reaction than the restricted group.

The increased allergic response on one test, but not the other, suggests that folic acid supplementation partially reduced the protection that has previously been seen in pregnancies with restricted growth. The results help scientists understand allergy risk in humans as well, the research team explained. “Patients should be counseled regarding the potential increase risks of progeny allergy of continuing folic acid supplementation for the entirety of pregnancy,” the researchers wrote.