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 Tylenol. However, I made the big mistake of googling “fever and 3.5 weeks pregnant,” and now I am freaking out about all sorts of things — neural tube defects, miscarriage, and even second-guessing my decision to treat it at all.

And, thinking ahead, does a fever only matter in early pregnancy, or do I need to worry about this for my entire pregnancy? 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 it was more commonly reported among the women whose children had birth defects.

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

This is scary — I wish the answer could be more comforting — and it’s not a completely dismissible 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.

Third, these impacts really apply to the first trimester of pregnancy, when these parts of the baby are developing.

Finally, there are options to treat a fever during pregnancy, which would be expected to lower any negative impacts. The recommended fever treatment is with Tylenol (acetaminophen). Despite some discussions to the contrary, the best data we have shows Tylenol usage to be safe in pregnancy. And this is a case where that treatment can prevent possible negative impacts from the fever itself.