Clinically, the normal number of weeks of pregnancy is generally considered to be 40 weeks. Calculation of pregnancy time is usually calculated from the first day of the last menstrual period, and a full-term pregnancy is usually 280 days, or about 40 weeks. The month here does not refer to the natural month in normal life, but is calculated as a month of 28 days. However, the expected time of delivery varies from person to person due to the fact that each person has a different constitution and the child develops differently. Normally pregnancy takes ten months, but there are some premature babies with less than ten months of gestation, and there are also some cases where the delivery takes place after the due date. Some women have irregular menstrual cycles, and there may also be some errors in determining the time of pregnancy based on the menstrual cycle.