The time difference between the last menstrual period and actual conception needs to be considered according to each person’s menstrual cycle, which varies greatly between women with regular and disordered menstrual cycles. 1. Regular menstrual cycle: If the menstrual cycle is usually regular, with a cycle of 28-30 days, the time difference between the last menstrual period and actual conception is usually 10-15 days, which is a normal situation. Since ovulation is about 14 days before menstruation, if the sperm and egg combine to form a fertilized egg on the day of ovulation, it will be about half a month before the last menstruation. 2. In this case, the actual gestational week can be estimated based on early ultrasound and laboratory tests. Later ultrasounds will also be performed periodically to check whether the fetal indicators are in accordance with the gestational week. Therefore, for pregnant women with irregular menstruation, the gestational week of the fetus as determined by the ultrasound is used clinically. The timing of the expected delivery date can also be based on the ultrasound projection of the expected delivery date.