Anything abnormal during pregnancy can cause a lot of anxiety in the mother, especially as she is concerned about the health of her baby. Fever during pregnancy can be one of the causes of her anxiety. A fever is an increase in body temperature. Fever happens when a person is sick and could be a sign of the body fighting against some kind of infection. The average body temperature is 98.6°F (37°C). For a woman who is pregnant, body temperature over 101°F (38.3°C) may be a cause for concern.