DEAR MAYO CLINIC: I'm newly pregnant and not looking forward to the terrible morning sickness I experienced with my first two pregnancies. There were weeks when it felt like I could not keep any food down. What causes morning sickness? Is there any way to prevent it? At what point should I be seen by a physician?

ANSWER: It's not clear what causes morning sickness -- the nausea and vomiting many women have during pregnancy. There are home remedies you can try that may help. If you start to lose weight, if you can't keep liquids down, or if vomiting becomes severe, see your doctor right away.

Although it is called morning sickness, that term is not accurate, as the symptoms can happen any time and, in some women, may last all day. Morning sickness is most common during the first trimester. But when it starts and how long it lasts can vary quite a bit. In a small number of cases, morning sickness can be an issue throughout pregnancy.

Doctors don't know exactly why women get morning sickness. The hormone changes that happen during pregnancy are thought to play a role. When pregnancy begins, a woman's body starts making a hormone called human chorionic gonadotropin (hCG). Morning sickness often kicks in when production of hCG begins. The production of hCG tends to be higher in twin pregnancies. The fact that morning sickness is more common in women carrying twins seems to reinforce the theory that hCG is connected to morning sickness.

Whatever the cause, morning sickness can be hard to prevent. There are ways you may be able to make it less bothersome, though. For example, nausea tends to be worse when your stomach is completely full or empty. So rather than eating three large meals a day, eat smaller amounts more often. Many women find that snacking on soda crackers or dry toast can quell feelings of queasiness. Drink plenty of fluids throughout the day, too. But don't drink too much at one time. Water and ginger ale are often good choices.

Limit the amount of greasy, spicy or fatty foods you eat, as they are more likely to cause nausea and vomiting. The smell of certain foods, especially during cooking, can be a problem for some women with morning sickness. Try to avoid using problematic foods if you're preparing meals, and enlist someone else to help make meals if cooking triggers nausea.

Also, pay attention to when and how you take your prenatal vitamins. Some women find that taking them in the morning makes nausea worse. If that's the case for you, try taking them at night. Having a snack, chewing gum or sucking on hard candy after you take your vitamins also may help. Taking a children's chewable multivitamin in place of prenatal vitamins may be an option, too.

If nausea and occasional vomiting continue, your doctor may suggest over-the-counter medications. A combination of doxylamine succinate, a sleep aid, and vitamin B6 often decreases symptoms. Both of these medications are safe in pregnancy. If that doesn't work, a prescription medication, such as promethazine or ondansetron, may be useful.

For most women, morning sickness is a nuisance that fades as pregnancy progresses. However, a small percentage of women develop serious nausea and vomiting, called hyperemesis gravidarum, which could threaten their health and possibly the health of the baby.

Women who have hyperemesis gravidarum often become dehydrated and lose weight. If it isn't treated quickly, hyperemesis gravidarum can lead to hospitalization. In the hospital, intravenous fluids and nutrition may be used to treat severe morning sickness. Rarely, hyperemesis gravidarum may result in premature birth or low birth weight. Fortunately, morning sickness often can be successfully managed -- even in more serious cases -- without long-term health risks to the mother or baby. -- Myra Wick, M.D., Ph.D., Obstetrics and Gynecology, Mayo Clinic, Rochester, Minn.

(Medical Edge from Mayo Clinic is an educational resource and doesn't replace regular medical care. E-mail a question to medicaledge@mayo.edu. For more information, visit http://www.mayoclinic.org.)