It's very common -- and normal -- to have an upset stomach when you're pregnant. Chalk it up to pregnancy's hormonal changes. It usually happens early in pregnancy, while your body is adjusting to the higher hormone levels. Good news: Nausea usually disappears by the fourth month of pregnancy (although in some cases it can persist throughout the pregnancy). It can happen at any time of the day but may be worse in the morning, when your stomach is empty (that why it's called morning sickness) or if you aren't eating enough.