The correction of the menstrual age is one of the more common obstetrical tasks that need to be done by the obstetrician. During pregnancy, a woman determines her gestational age based on her last menstrual period, but there are important prerequisites for this calculation, namely a regular menstrual cycle of about 28 days. If a woman fails to meet this condition, or ovulation is affected, it is inaccurate to speculate the time of pregnancy based solely on the last menstrual period, so that relevant corrections need to be made. Generally speaking, the age of the month can be corrected according to the ultrasound of early pregnancy, and the size of the gestational sac, the length of the embryonic bud and the diameter of the head and buttocks can be measured during early pregnancy to determine the theoretical time of the last menstrual period, and then estimate the expected date of delivery and the actual pregnancy gestational week.