Morning sickness most commonly starts around the sixth week of pregnancy, but some people may begin to feel nausea as early as the fourth week, which is just about two weeks after conception. Symptoms usually start mildly around week 6, peak around weeks 9 to 10, and often improve by weeks 12 to 14, near the start of the second trimester. However, experiences vary widely, and some may have symptoms earlier or later into pregnancy.