The hormone called human chorionic gonadotropin, or HCG, is produced by the fertilized egg and by the chorionic villi. These are the fingerlike projections of the developing placenta. HCG is needed to keep the pregnancy going until the placenta has developed enough. HCG levels are usually highest in the first 12 weeks of pregnancy. There is a huge variation in how bad the symptoms of morning sickness are.
Some women just feel queasy in the morning, and feel fine by noontime.
They are able to perform their daily activities. Others suffer from nausea and vomiting that can last all day. They find ordinary work during pregnancy almost impossible. Some women have symptoms of morning sickness within days of conception. But the average time from the last menstrual period to the start of nausea is about 5 weeks.