The time of pregnancy is not calculated from the date of intercourse, but from the first day of the last menstrual period. Although the last menstrual period is not the date of ovulation or the date of fertilization, the length of the menstrual cycle, the time of ovulation, and the time of formation of the fertilized egg vary from woman to woman, so it is agreed to use the last menstrual period as the starting point in calculating the time of pregnancy. However, since the date of ovulation is about two weeks later than the last menstrual period and the time of fertilization is usually three weeks later than the last menstrual period, the accounting time may not match the first day of the last menstrual period, so it needs to be corrected in conjunction with the in-hospital examination of the pregnant woman.