There are two ways of calculating pregnancy, either by using the last menstrual period or by ultrasound. The two methods of calculation will usually be compared and if there is a large difference between the two, the ultrasound calculation method is more accurate. 1. Last menstrual period: For women with regular periods, the number of days of pregnancy can be calculated from the first day of the last menstrual period to the present time. 7 days is one week, which can be used to calculate the number of weeks of pregnancy, and 4 weeks is one month, which can be used to calculate the month of pregnancy. 2. Ultrasound: If a woman has irregular periods or does not remember the time of her last menstrual period, the method of calculating the number of weeks of pregnancy using the last menstrual period is not accurate enough. Ultrasound can be used to determine the number of weeks of pregnancy, which can then be used to project the time of the last menstrual period. Pregnant women need to calculate the exact time of pregnancy, so that they can judge the development of the fetus against the weeks of pregnancy during the delivery test.