Inaccurate menstruation may cause some errors in the calculation of the expected date of delivery, which can be determined by early ultrasound to determine the number of days of pregnancy. In about 50% of pregnant women, inaccurate menstruation causes inaccuracy in projecting the due date through the last menstruation, which can be corrected by early ultrasound. Early ultrasound refers to the small 4D examination done between 11 and 13 weeks. In addition to measuring NT thickness, the small 4D can also measure the top-rump diameter of the fetus, and it is more accurate to estimate the week of pregnancy through the top-rump diameter of the fetus. If the gestational week measured by the 4D is more than 1 week different from the actual gestational week of menopause, the results of the 4D ultrasound should prevail. For pregnant women with irregular menstruation, if you find that the size of the fetus does not match the gestational week, you must check the gestational week again to avoid inaccurate calculation of the gestational week, which may lead to the occurrence of full-term premature babies.