1. How long does it take to become pregnant before symptoms of morning sickness appear? How long in pregnancy will symptoms of morning sickness occur is a question that every expectant mother is concerned about. Generally speaking, morning sickness is a common symptom in early pregnancy. It begins to appear around 5-6 weeks of pregnancy and gradually disappears until about 12 weeks. However, some expectant mothers will experience morning sickness for a longer period of time. 2. Why does morning sickness occur? Morning sickness is caused by the influence of hormones such as human chorionic villi (HCG) and progesterone secreted by the placenta.