How Dangerous Is a Fever In Early Pregnancy?

Recently found out I’m pregnant (yay), but I’m also newly ill. I think my fever (yesterday, before I found out I was pregnant) only lasted a few hours, and I treated it with medicine. However, I made the big mistake of googling “fever and 3.5 weeks pregnant” and now I am freaking out about neural tube defects, cleft palates, and miscarriage. I understand you can’t tell me if anything is actually wrong with my baby, but it would help me to wrap my head around the data. I wish I could just be happy about my big news.
—Two tests in one day

Congratulations on your pregnancy, and I’m sorry it has started out scary (and sick).

There are associations in the data between temperature increases in early pregnancy and a small number of birth defects, including neural tube defects and cleft palate. The most comprehensive data on this comes from a 2018 paper that surveyed 17,000 women whose children had birth defects, and 10,000 whose children did not. The researchers asked the women about fever (among other things) and looked at whether fever was more commonly reported among the women whose children had birth defects.

The study found an elevation in several birth defects, ranging between a 20% and a 300% increase.

This is scary — I wish this answer could be more comforting — and it’s not a completely dismissable risk. We have other data that also suggests that raising body temperature significantly for longer periods may be a risk factor for these birth defects.

However: there are a few things to emphasize here. First, the increases are very small in absolute numbers, because these birth defects are rare. So when we see a “50% increase,” that sounds large, but it’s not very many cases. Moreover, a huge share of women — 6% to 8% — have fevers in early pregnancy. The vast, vast majority of them do not have babies with birth defects. It’s easy to see a “significant effect” and think that it is important. This is a case that may be significant but small.

Second, the effects in these papers generally seem to scale with severity. A few hours of a mild to moderate fever is on the very low end, so even the small increased risk is likely a huge overestimate.

The perhaps good news is that you should know fairly early in your pregnancy whether there is more to be worried about, and many of the issues seen here are treatable.