It isn't easy being green, especially for the nearly three-quarters of all newly pregnant women who experience nausea during pregnancy. And despite its name, "morning sickness" can strike at any time.
Pregnancy nausea usually begins between four and eight weeks of pregnancy. There's no one explanation for what causes it, says Jennifer Niebyl, M.D., professor of obstetrics and gynecology at the University of Iowa, in Iowa City. Researchers currently suspect a variety of factors including levels of pregnancy hormones such as human chorionic gonadotropin (hCG), certain gastrointestinal bacteria like H. pylori, and vitamin deficiency (particularly vitamin B6). "The hormone hCG plays a fundamental role in morning sickness," says Dr. Niebyl. "If you're carrying multiples, you'll experience more nausea because of increased hCG levels."