Common signs and symptoms of morning sickness include nausea and vomiting, often triggered by certain odors, spicy foods, heat, excess salivation or — often times — no triggers at all. Morning sickness is most common during the first trimester and usually begins by nine weeks after conception. Symptoms improve for most expectant mothers by the mid to late second trimester.

What causes morning sickness isn't clear, but the hormonal changes of pregnancy are thought to play a role. Rarely, severe or persistent nausea or vomiting may be caused by a medical condition unrelated to pregnancy — such as thyroid or liver disease.

Morning sickness can affect anyone who's pregnant, but it might be more likely if:

• You had nausea or vomiting from motion sickness, migraines, certain smells or tastes, or exposure to estrogen (in birth control pills, for example) before pregnancy
• You had morning sickness during a previous pregnancy
• You're pregnant with twins or other multiples

You might be more likely to experience hyperemesis gravidarum if:

• You're pregnant with a girl
• You have a family history of hyperemesis gravidarum
• You experienced hyperemesis gravidarum during a previous pregnancy

Mild nausea and vomiting of pregnancy typically won't cause any complications to you or your baby.

If left untreated, severe nausea and vomiting can cause dehydration, an electrolyte imbalance, decreased urination and hospitalization. Research is mixed on whether hyperemesis gravidarum causes poor weight gain for your baby during your pregnancy.