Women who were healthy at baseline had an increased risk of atrial fibrillation if they had multiple pregnancies, according to an analysis of a large cohort.

Lead researcher Jorge A. Wong. MD, MPH, of the Population Health Research Institute at McMaster University in Hamilton, Ontario, Canada, and colleagues published their results online in Circulation on Feb. 2.

The researchers analyzed data on 34,639 women who enrolled in the Women’s Health Study, which initially began as a randomized trial comparing aspirin versus placebo for the primary prevention of cardiovascular disease and cancer. After the trial was completed in March 2004, participants were asked to participate in an observational follow-up.

The women were 45 years old or older and were free of cardiovascular disease and atrial fibrillation at baseline. They reported their number of pregnancies lasting at least six months at baseline. They also reported their incident atrial fibrillation events at 48 months of follow-up and every year afterward. The researchers only included atrial fibrillation events that they could confirm by medical record review.

At baseline, the median age was 52.9 years old and the median number of pregnancies was two. During a median follow-up period of 20.5 years, there were 1,532 incident atrial fibrillation cases.

After the researchers adjusted for age, they found that an increasing number of pregnancies increased the risk of atrial fibrillation. They added that the association was strengthened after they controlled for body mass index, diabetes, other atrial fibrillation and cardiovascular disease risk factors, reproductive factors and socioeconomic status. In addition, they mentioned that the number of cardiovascular events during the follow-up period did not affect the association between pregnancies and atrial fibrillation risk.

The researchers cited a few limitations of the study, including that most of the women were of European descent, so the results might not be generalizable to women of all races and ethnicities. The women also self-reported their number of pregnancies and atrial fibrillation. In addition, they noted that the results could be subject to residual confounding.

“The point here is not to discourage women from having children,” Wong said in a news release. “However, our research highlights that something about pregnancy predisposes women to this greater risk, and more research is needed to help us understand why.”