The pregnancy cycle is usually counted from the first day of the last menstrual period, and for women with normal menstrual cycle, the due date and gestational week are calculated from the first day of the last menstrual period, the due date is the month of the last menstrual period minus 3 or plus 9, and the date is the date of the last menstrual period plus 7. For women with irregular periods, the week of pregnancy is estimated during the first ultrasound examination according to the size of the gestational sac, and according to the first estimated week of pregnancy in turn, the normal time of delivery is between 37 and 40 weeks. Pregnant women should have a checkup every four weeks during pregnancy, and after 36 weeks of pregnancy, they need to have a weekly checkup, and prenatal checkups such as Down screening and 4D should be done according to the pregnancy week, and the number of prenatal checkups should be increased for high-risk pregnant women.