Morning sickness, also called nausea, vomiting of pregnancy (emesis gravidarum or NVP), or pregnancy sickness, affects more than half of all pregnant women, as well as some women who use hormonal contraception or hormone replacement therapy. The nausea can be mild or induce actual vomiting. In extreme cases, known as hyperemesis gravidarum, hospitalization might be required to treat the resulting dehydration, which occurs in about 1% of pregnancies.

Duration of condition

Morning sickness can occur at any time of the day, though it occurs most often upon waking,cn: date=October 2008 because blood sugar levels are typically the lowest after a night without food.

Morning sickness usually starts in the first month of the pregnancy, peaking in the fifth to seventh weeks, and continuing until the 14th to 16th week. For half of the sufferers, it ends by the 16th week of pregnancy. It might take the others up to another month to get relief, and some women suffer intermittent episodes throughout their pregnancy.

Causes

There is insufficient evidence to find a single (or multiple) cause, but the leading theories for proximate causes include:

• An increase in the circulating level of the hormone estrogen. Estrogen levels may increase by up to a hundredfold during pregnancy. However, there is no consistent evidence of differences in estrogen levels between women who experience sickness and those who don't.
• Low blood sugar (hypoglycemia) due to the placenta draining energy from the mother. Though studies have not confirmed this.
• An increase in progesterone relaxes the muscles in the uterus, which prevents early childbirth, but may also relax the stomach and intestines, leading to excess stomach acids and Gastroesophageal reflux disease.
• An increase in human chorionic gonadotropin.
• An increase in sensitivity to odors, which overstimulates normal nausea triggers.
• An increase in bowel movement.
• The body's effort to detoxify thoroughly for the health of the unborn child. This is supported by evidence that the liver and kidneys become more active than usual at the start of a pregnancy.

As for root causes, this issue is still somewhat controversial. A notable current scientific hypothesis is that morning sickness exists as a safeguard for the embryo's health. Biologists Gillian V. Pepper and S. Craig Roberts have done a study that indicates that the intake of alcohol, sugar, oils, and meat can trigger morning sickness. This then acts as a way of discouraging ingestion of less healthy foods.

According to Margie Profet, eating vegetables might be a factor as well, due to their small amount of toxins to deter insect infestation; while these toxins are normally harmless to adult humans, they are potentially dangerous to embryos. However, this idea has been rejected by a prospective, population-based study which concluded that "claims made in the popular press about food and health relationships should be evaluated by the media as fiction unless supported by scientific research". Both Profet's vegetable theory and Deutsch's suggestion morning sickness's role is to reduce frequency of sexual intercourse, so preventing sexual uterine cramping that might be a cause spontaneous abortion, have been rejected by a cross-cultural study that suggested morning sickness is more frequently observed in societies that have animal products as dietary staples which may "be dangerous to pregnant women and their embryos because they often contain parasites and pathogens" and hence "that morning sickness serves an adaptive, prophylactic function".