During the fifth week of my pregnancy, I got the flu. There was no significant fever and it lasted for few days. Does the influenza virus, or other viruses of the common cold, cause fetal malformations? Are they teratogenic?

O.

First, let me explain that you probably did not have influenza. Classic influenza includes the abrupt onset of fever, severe muscle aches and upper respiratory symptoms. Influenza virus can cause the common cold, but other viruses, especially rhinovirus, are much more likely to be to blame in this setting.

Infection in pregnancy is a very difficult subject to study. First, it is hard to find a group of pregnant women who definitely have the infection being studied. A researcher cannot simply ask women if they had influenza or rhinovirus or measles. Instead, the diagnosis has to be proven. Then, medical researchers have to compare these women and their pregnancy outcomes with women who are similar in every way except the infection. In other words, to study the effect of influenza on pregnancy, one needs a "control group" of pregnant women who did not have influenza. One will find that in the control group, some women will have miscarriages, and a few babies will be born with malformations. If a control group was not included in the study, one might erroneously think that all problems in the influenza group are due to the infection.

Evidence shows that some infectious diseases are highly likely to cause fetal malformations. These include toxoplasmosis, rubella, cytomegalovirus, herpes, syphilis, parvovirus and listeriosis. Other diseases have been shown to result in miscarriage and possibly malformation, but at a rate only slightly higher than in a control group. These include varicella (chickenpox), measles and mumps. This list is by no means complete. There are many other infectious agents that have been shown to result in fetal infection, and many others that have not been adequately studied.

Upper respiratory viruses such as influenza, rhinovirus and others are extremely common in pregnancy. Since most people have a few such infections a year, it follows that most pregnant women will have at least one. The vast majority of these infections result in no adverse effects for the mother or the fetus. Therefore, there is no recommendation for any medical intervention with the usual upper respiratory infections.

As to influenza specifically, there are a few studies linking exposure in the womb to the risk of mental illness in adulthood. The risk of mental illness is low, but it may be slightly higher than in people who were not exposed to influenza in the womb. The same has been shown in a few small studies linking influenza exposure in the womb to the future risk of brain tumors. I want to stress, however, that the increased risk is small. The vast majority of people who were exposed to influenza during the fetal period have no ill effects.