The gestation period is calculated from the first day of the last menstrual period, and the full gestation period is about 280 days, or about 40 weeks. The clinical period is artificially divided into 3 periods according to the different time periods of pregnancy, early pregnancy, middle pregnancy and late pregnancy. Early pregnancy refers to the period from the beginning of pregnancy until it reaches 14 weeks, so 13+6 weeks is early pregnancy, 14-27+6 weeks is called middle pregnancy, and 28 weeks and beyond is called late pregnancy. Early pregnancy, also known as early pregnancy, is an important period for embryo formation and fetal organ differentiation. Middle and late pregnancy are important periods for fetal growth and maturation of various organs, and most fetal developmental abnormalities and developmental abnormalities are determined by ultrasound and various related examinations during this time period.