In clinical terms, the pregnancy cycle is calculated from the first day of the last menstrual period and works backwards in order, with each day that passes increasing the number of days of pregnancy by one day. One week of pregnancy means seven days, and the expected date of delivery is until the 40th week of pregnancy. If a pregnant woman forgets her last menstrual period, the week of pregnancy can be calculated by ultrasound. The number of days of pregnancy should be counted from the day of conception, but since this date is difficult to determine, the first day of the last menstrual period is considered the beginning of the pregnancy cycle in clinical practice. Most women can remember their last menstrual period clearly, so this method of calculating the week of pregnancy does not usually lead to errors. However, if a pregnant woman cannot remember the date of her last menstrual period, she can also determine the week of pregnancy by ultrasound in early pregnancy. If there is a difference between the two methods, ultrasound should be used as the main indicator. The pregnancy cycle is calculated in order to project the expected date of delivery in advance. Under normal circumstances, the whole pregnancy cycle is 40 weeks, counting from the first week of pregnancy and pushing back 40 weeks is the so-called due date. In short, accurate knowledge of the week of pregnancy not only allows you to prepare for the birth in advance, but also allows you to determine whether the fetus is developing normally according to the week of pregnancy.