Morning sickness is nausea or vomiting during the first 20 weeks of pregnancy. More than half of pregnant women have morning sickness during the first trimester. It usually goes away by the second trimester. When morning sickness is severe, it is called hyperemesis gravidarum.

What is going on in the body?

The cause of morning sickness is not well understood, but hormones seem to be involved. The hormone called human chorionic gonadotropin, or HCG, is produced by the fertilized egg and by the chorionic villi. These are the fingerlike projections of the developing placenta. HCG is needed to keep the pregnancy going until the placenta has developed enough. HCG levels are usually highest in the first 12 weeks of pregnancy.

What are the causes and risks of the condition?

A woman with high levels of HCG is more likely to have morning sickness. High levels of HCG are seen in multiple pregnancies, such as twins and triplets. A woman who has had morning sickness in a previous pregnancy is more likely to have it again.

Increased HCG levels can be caused by a molar pregnancy, or tumor of the placenta. This condition should be ruled out in women with morning sickness. There is some evidence that psychological factors, such as ambivalence toward pregnancy, can increase the risk of morning sickness.