According to New Health Guide, morning sickness starts after conception in the first trimester of pregnancy as early as the second week of a woman's pregnancy. Approximately 50 percent of pregnant women experience some symptoms of morning sickness.
Morning sickness may be the first indicator of pregnancy, so most women take a pregnancy test if they are experiencing nausea or vomiting in addition to a missed menstrual cycle. New Health Guide explains that most women find morning sickness to be diminishing by the 12th week of pregnancy, but some women experience morning sickness until the 16th week. Morning sickness can reoccur during the third trimester of pregnancy for some women as well.