For most pregnant women, morning sickness starts around the 6th week of being pregnant, according to BabyCenter. For some women, it can start as early as four weeks into a pregnancy.
Regardless of when a specific woman begins to feel the effects of morning sickness, it usually gets worse in the month after first experiencing symptoms, explains BabyCenter. Common symptoms include nausea and vomiting, although about 25 percent of women experience just one of those side effects. Another 25 percent report no side effects at all.
By week 14 of a pregnancy, most women experience complete relief of symptoms, according to BabyCenter. For those that do not experience relief after 14 weeks, it usually takes an additional four weeks for symptoms to vanish. A small percentage of women continue to have side effects of morning sickness past this point, which usually lasts until the baby is delivered or shortly before delivery.
As of 2015, the cause of morning sickness is still not understood, states BabyCenter. However, scientists theorize that it may be a result of all the physical changes a woman undergoes as a pregnancy develops. One possible contributing factor is the increase in the hormone called human chorionic gonadotropin. This hormone increases rapidly early in a pregnancy and peaks right around the time the woman begins to experience symptoms of morning sickness. Additionally, circumstances that further increase the hormone, such as being pregnant with multiples, subsequently increase the feeling of nausea in some women.