If you’ve ever been pregnant, you have probably experienced some form of morning sickness. I’ve only ever had one friend that never experienced morning sickness, but even she and her doctor were amazed, especially because she was a high-risk pregnancy. It’s been hypothesized that morning sickness protects the embryo by causing pregnant women to physically expel and subsequently avoid foods that contain teratogenic and abortifacient chemicals, especially toxic chemicals in strong-tasting vegetables, caffeinated beverages, and alcohol.

Despite numerous different hypotheses, no one actually knows for sure what causes morning sickness. However, what doctors have noticed is that some women are more likely to experience the symptoms of morning sickness than others. In fact, you’re more likely to experience morning sickness if any of the following apply to you:

• You're pregnant with twins or triplets. Doctors speculate that this may be from the increased levels of hCG, estrogen, or other hormones in your system.
• You experienced vomiting or nausea during a previous pregnancy.
• You have experienced nausea or vomiting because of taking birth control pills. This is probably due to your body's response to estrogen.
• You have a history of motion sickness.
• You have a genetic predisposition to nausea during pregnancy. If your mother or sisters have experienced severe nausea and vomited during pregnancy, then you’re more likely to experience the symptoms as well. 
• You have a history of migraine headaches.
• You're having a girl. One study found that women with severe nausea and vomiting in the first trimester are 50 percent more likely to be carrying a girl.

Also, morning sickness isn’t correctly named because it can happen all through the day. Some women experience nausea in the mornings, but it is just as likely to occur in the afternoon or evening. The technical name for morning sickness is actually nausea and vomiting of pregnancy. Other things that have been known to cause morning sickness include stress, a sensitive stomach, and sensitivity to smells.

To relieve the symptoms of morning sickness, obstetricians and midwives recommend eating small meals and snacks throughout the day instead of larger meals, avoid lying down after eating, getting up slowly after waking up, and avoiding fatty foods while trying to eat colder foods. Unfortunately, there is no cure for morning sickness. If your symptoms become severe and nothing works to ease them, then make an appointment with your doctor since severe nausea and vomiting can eventually have other adverse effects.

Source: Flaxman, S. M., & Sherman, P. W. (2000). Morning sickness: a mechanism for protecting mother and embryo. The Quarterly Review of Biology, 75(2), 113-148.

• PubMed