In general, the last menstrual period is used to calculate the weeks of pregnancy for those who have regular periods, but for those who have irregular periods, this method may have a large error, and such pregnant women are recommended to undergo an ultrasound examination to determine the weeks of pregnancy according to the development of the fetus. For those who have regular periods, the gestational week can be calculated based on the first day of the last menstrual period, and every 7 days is one gestational week, and 4 gestational weeks are one month, until the mother gives birth. For example, if the first day of the last menstrual period is June 1 and the first day of August, a total of 92 days, divided by 7 is approximately 13.1, which means that you are in the 14th week of pregnancy. Since ovulation and fertilization usually occur around the 14th day of the last menstrual period, the fact that an embryo has not formed before the 14th day cannot be considered as pregnancy, but only as the first trimester, so it is not accurate to calculate the week of pregnancy simply based on the last menstrual period. For those who have unclear or irregular last menstruation, in addition to ultrasound, the gestational week can also be calculated based on the measurement of uterine height, such as 18.3 cm for 20 weeks of gestation, 23.6 cm for 24 weeks of gestation, and 26.1 cm for 28 weeks of gestation. if the results of the above tests do not match the actual situation, it is necessary to consult a doctor for further examination. If the results of the above examination do not correspond to the actual situation, it is necessary to consult a doctor for further examination to identify the cause and, if necessary, treatment.