The exact cause of morning sickness is unknown. Many physicians believe that hormones play a role. During pregnancy there is a sudden increase in hormones. Rising levels of human chorionic gonadotropin (HCG), a hormone secreted by the fetus, is considered a factor. Rising levels of the female hormone estrogen levels have also been implicated. Some experts believe pregnancy hormones cause nausea and vomiting by relaxing the stomach.

The hormone theory is also supported by the fact that morning sickness usually disappears by the second trimester, the time the levels of pregnancy hormones fall. In addition to hormones, lower blood sugar during early pregnancy has been considered a potential cause of morning sickness.

Many researchers believe that hormones and morning sickness played an important role during human evolution. A common theory is that morning sickness during pregnancy protects the fetus by preventing the mother from eating foods that would be harmful to the pregnancy. This idea is supported by the fact that many of the foods that commonly repulse pregnant woman contain potentially harmful substances.

In addition, some studies have indicated that women who have little or no nausea or vomiting during pregnancy seem to be more likely to miscarry than those who experience some sickness. Many physicians believe morning sickness is a positive sign because it signals that the afterbirth (the placenta and fetal membranes) is developing properly. However, this link is not certain, and many women who experience little or no morning sickness have normal pregnancies.

In some women, an increased sensitivity to odor may also contribute to morning sickness. Pregnancy may cause the development of aversions to certain foods such as coffee or meat. The dislike for these foods is often associated with their odor. Many women experience an enhanced sense of smell during their pregnancy. This can result in these women becoming more sensitive to odors, triggering nausea and vomiting. Pregnant women may also experience changes in the taste of some foods.