Zofran and Your Baby's Heart

The cells that will become your baby’s heart are among the first to be formed after a pregnancy has reached the vertebrate embryo stage. The heart is the first organ to fully form and function during vertebrate development. This process takes about ten weeks—or 70 days-- before the walls of the lower chambers of the heart (the ventricular septa) are formed.

With this 70-day period in mind, let’s consider the conclusions of a February 2013 article in the New England Journal of Medicine (NEJM). In that paper, the Danish authors stated that Zofran (ondansetron) taken during pregnancy was not associated with a significantly increased risk of fetal outcomes, including heart defects. At first blush this statement might appear reassuring.

A second closer look calls into question the reliability of the authors’ findings as it relates to your baby’s heart.

Of all the pregnancies considered in this study, the middle (median) time when the mother’s first prescription for Zofran (ondansetron) was filled was at 70 days of pregnancy. If that is true, then the septal walls of the babies’ hearts were already formed in as many as half of the pregnant women who were taking Zofran. Wouldn’t that tend to skew the data in favor of the conclusion that this drug is safe to take during pregnancy?

It is interesting that within months of publication of the NEJM paper, another group of Danish researches came to an opposite conclusion when they focused on a larger group of women who redeemed a prescription for Zofran (ondansetron) during their first trimester, that is, before the first 70 dats of pregnancy. Doesn’t this type of study make more sense?

And the conclusions of this more focused study arrived at the opposite result: They found an increase in the prevalence of major congenital heart defects in children whose mothers redeemed a prescription for Zofran (ondansetron) in the first trimester of pregnancy. In fact, when they looked at the prevalence of an atrial or ventricular septal wall defect, the increased risk of such malformations was almost five times (4.8) that of a woman who did not take Zofran (ondansetron) during her first trimester. It helps to closely scrutinize the data before concluding that a drug such as Zofran is safe to take during pregnancy.


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