Nausea during pregnancy, commonly called “morning sickness”, is experienced by about four out of five women. Although called morning sickness, it may occur at any time during the day or night. The causes of morning sickness are not fully understood. This nausea and sometimes vomiting is believed to be related increased levels of a hormone produced by the placenta called human chorionic gonodatropin (HCG). You may take comfort in the fact that most nausea disappears by the 16-18th week.