Most cases of morning sickness are annoying -- but not harmful. But if you're throwing up so much that you can't keep liquids down or if you're not urinating, you need to let the doctor know right away. "This can lead to severe dehydration, which isn't good for you or your baby," says Isabel Blumberg, MD, an ob-gyn in New York City. It can also be a sign that you're suffering from hyperemesis gravidarum -- a type of extreme morning sickness that can last throughout your entire pregnancy. Also call if you haven't been able to keep food down for two days straight, if you think you have food poisoning, or if the vomiting is accompanied by a high fever. In these cases, you may need to go to the hospital for IV fluids.