Nausea is a common symptom of pregnancy--three-quarters of all pregnant women will experience some nausea or vomiting or both, especially during the first trimester. Nonetheless, despite its frequency, nobody knows quite what causes it.

Misconceptions: It's Not Morning Sickness

Nausea during pregnancy is colloquially known as 'morning sickness,' but the fact is that it can strike any time during the day or night. Some people fear that nausea during pregnancy is a sign that something's wrong with the pregnancy, or a signal that it's higher-risk than normal. There is, however, no evidence that this is the case and some researchers even theorize the opposite, that higher-risk pregnancies actually exhibit less nausea than average.

hCG

One leading theory about the cause of pregnancy-related nausea is that it's caused by elevated levels of the hormone human chorionic gonadotropin (hCG). hCG is produced only during pregnancy; in fact, it's the same hormone that pregnancy tests look for. hCG's role in pregnancy is to support the ovary's production of progesterone during pregnancy. It may also play a role in shielding the developing fetus from the mother's immune system.

Sense of Smell

A pregnant woman's sense of smell typically becomes heightened and highly sensitive during pregnancy. Sometimes, the heightened odors she detects will trigger a wave of nausea or even gagging. The causes and mechanisms of this heightened sensitivity are unknown, but some scientists speculate that it has a protective effect, allowing women to avoid questionable food.

Sensitive Stomach

Some women's digestive tracts are more sensitive during pregnancy. This may be linked to the presence of a bacteria known as H. pylori (which can also ulcers). Women with more of this bacteria may be more likely to experience pregnancy-related nausea. Certain pregnancy hormones may also relax the muscles of the stomach and thus trigger nausea.

Stress

Psychological stress may also be a trigger. If a woman is in a stressful job, or has a stressful home life for whatever reason, pregnancy-related nausea may follow.

Other Factors

If a woman has experienced nausea in a previous pregnancy, she's more likely to have it again with a subsequent one; if her mother or sisters ever experienced it, her chances go up, too. Women expecting twins or multiples are more likely to have pregnancy-related nausea. Women with a medical history of migraines or motion sickness are more prone to this condition. And, according to at least one study, carrying a girl may increase the chance that a woman experiences nausea during her pregnancy.

Source:

BabyCenter.com: Morning Sickness

Ob-Gyn.net: Nausea and Vomiting During Pregnancy