Fever in the first trimester

In the first trimester, I ran a temperature ranging from 99 to 103 degrees for nearly four days, thanks to a nasty bout with the flu. I read that a fever in the early weeks could harm the fetus. However, it didn't state the chances or the type of harm. Can you tell me?


Peg Plumbo CNM

Peg Plumbo has been a certified nurse-midwife (CNM) since 1976. She has assisted at over 1,000 births and currently teaches in the... Read more

There is very little research on the effect of fever on the fetus. For one reason, a controlled study is impossible because a researcher cannot expose pregnant women to heat and study the outcome a few months later. So, all we know about this topic is from animal studies, case reports or clusters of data on maternal fever with differing causes.

A second reason is that it is difficult (impossible) to separate the reason for the fever from the actual hyperthermia. In other words, was it the flu or the fever which caused the problem.

Another reason is that babies are all exposed at various gestational ages to various insults such as fever. At one critical period, a baby might not tolerate it as well as he might at another time. So it is hard to compare all babies across all weeks, even when grouped into the first, second or third trimester.

It does appear that fever CAN cause anomalies in the central nervous system but it does not always. Babies at particular risk for cerebral palsy, for example, are those exposed to high temperatures around the time of labor (but it could also be the bacteria which causes the problem). Neural tube defects are more common in mothers exposed to fever in the first trimester. A triple screen or AFP test could help detect this condition.

In M. Paul's book: "Occupational and Environmental Reproductive Hazards", she says that when maternal core temps reach 102 degrees F. (38.9 C. or greater), there have been reports of abnormal fetal development. Fevers this high, which occur during the period of neural tube closure (days 20 to 28 after conception), have been associated with a significantly increased incidence of central nervous system related defects.

Studies of influenza epidemics in Europe in the 1950s and 1960s reveal a two to three fold increase in congenital anomalies among infants born to women with sustained fever episodes in the first trimester. No fetus' exposed to high temps after 16-weeks-gestation (18 weeks by LMP) had malformations. There were no stillborn or premature births reported. In these studies, all fevers were above 102 degrees and these were sustained for one or more days.

In another study, no association was found between febrile episodes and incidence of birth defects or problems with the pregnancy. "Other prospective and retrospective studies failed to find a significant association between hyperthermia due to influenza in early pregnancy and abnormal pregnancy outcomes" (M. Paul).

I hope this does not cause you to worry throughout the remainder of your pregnancy. The body's ability to regulate temperature is truly remarkable. It is unlikely that you maintained a significantly damaging temperature for a long period of time. Many mothers suffer from fever, upper respiratory infections and flu during their pregnancies and give birth to very healthy babies.

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