Based on the last menstrual period which is usually counted from the first day of the last menstrual period and then counting backwards, the number of days obtained is divided by seven to arrive at the week of conception. In general, ovulation usually occurs about 14 days before the next menstrual period in women with regular periods. After the eggs are released, they can survive in the body for about 1-2 days. Sperm enter the woman’s body and can only survive for 1-3 days. If you have intercourse during this period, the probability of pregnancy is relatively high. The pregnancy week projection is also relatively accurate. If a woman’s menstrual cycle is irregular and the time of ovulation is not fixed, the accuracy of projecting the week of pregnancy based on the last menstrual period is low, and the week of pregnancy should be projected based on the size of the ultrasound measurement. In medicine, the week of pregnancy is usually calculated according to the first day of the last menstrual period, which is a relatively simple and highly practical method of calculation.