Part One: Basic Knowledge of Pregnancy To calculate the time of conception, you need to first understand the female menstrual cycle and daily process as well as the basic process of conception. The first day of the menstrual cycle refers to the first day of menstrual bleeding. The cycle is usually about 28 days, but some women\’s cycles are irregular. The process of conception refers to the combination of sperm and egg to form a fertilized egg, which then develops into an embryo in the fallopian tube and finally implants in the inner wall of the uterus to form a pregnancy. 1.1 Calculation of menstrual cycle To calculate menstrual cycle, you need to rely on menstrual records. Record the first day of menstruation until the next menstrual period