The time difference between the last menstrual period and actual conception needs to be considered according to each person’s menstrual cycle, and there is a big difference between women with regular menstrual cycles and those with disturbed menstrual cycles.1. Regular menstrual cycle: if the menstrual cycle is usually more 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. Because the time of ovulation is about 14 days before the onset of menstruation, so 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 menstrual period; 2. Disordered menstrual cycle: If the menstrual cycle is not regular, or the menstruation is disordered, the time of the last menstruation and the actual time of conception can be completely inconsistent with each other, that is, it is not possible to estimate the actual week of conception according to the last menstruation. In other words, the actual week of conception cannot be estimated from the last menstrual period, and the actual week of conception can be estimated from the early ultrasound and laboratory tests. Later ultrasounds will also be used for regular evaluations to check whether the fetus’s indicators are consistent with the week of conception. Therefore, for pregnant women with irregular menstruation, the week of gestation of the fetus checked by ultrasound will be used as the basis for the clinical diagnosis. The expected date of labor can also be based on the ultrasound projected due date.