The egg survives for 12-24 hours after expulsion, but fertilization usually occurs within 12 hours after the egg is expelled, and ovulation usually occurs 14 days before the next menstrual period. For women with regular menstruation, pregnancy and contraception can be known roughly by projecting the ovulation period. The sperm can survive in the female reproductive tract for 2-3 days and the egg can survive for 12-24 hours after ovulation. Therefore, if a woman wants to get pregnant, she should choose to have intercourse around the time of ovulation, usually 3 days before and within 24 hours after ovulation, which will increase the chance of conception. For contraception, it is best to avoid the period before and after ovulation, which is commonly known as the safe period, when the chances of pregnancy are lower.