Fertility awareness (also called natural family planning or FAM) is a method to check the regular changes in a woman's body as it goes through during a menstrual cycle. This information can help you learn when you are fertile, if and when you ovulate, and when to have intercourse to improve your chances getting pregnant (or not getting pregnant).
A woman is usually able to get pregnant for about 5 days each month, the day of ovulation and the 4-5 days before the day of ovulation. On average, ovulation occurs 14 days before the next menstrual period begins. So ovulation would occur on about day 14-15 of a 28-day menstrual cycle, or day 21 of a 35-day cycle. Sperm can live for 4 to 5 days in a woman's reproductive tract, so it is possible to become pregnant if sex occurs up to 4-5 days before ovulation, but yoiur best chances getting pregnant is when you have intercourse the day of ovulation and the 1-2 days prior to that day.