The safe period is 1 day before a woman’s period, during which ovulation has already occurred and the corpus luteum has shrunk, and the estrogen and progesterone in the body have been reduced, when the endometrium has begun to peel off. Under normal circumstances, a woman’s menstrual cycle is around 30 days, and ovulation is in the middle of two periods, i.e. around the 14th day before the next period, while the days just after menstruation and the days before menstruation are called the safe period, during which intercourse is usually without egg discharge, so the sperm cannot combine with the egg to develop into a fertilized egg, and the woman will not become pregnant. In rare cases, ovulation occurs one day before menstruation and intercourse happens during this period, so pregnancy may occur.