Understanding menstrual physiology is the basis of understanding the whole concept of fertility, including the mechanism of action of contraceptives. It is also the basis for understanding natural family planning (NFP) methods. The corpus luteum has an inherent life span of about 14 days, and as it succumbs, in the absence of a pregnancy, the levels of oestrogen and progesterone decline. To maximize the chance of fertilization, sperm need to be deposited into the vagina prior to ovulation. It is believed that sperm will survive for two to three days in the mucus environment, so couples who want to conceive are advised to undertake intercourse at least every second day from when menstruation finishes until ovulation is thought have occurred. There have been a number of modifications of NFP as the understanding of reproductive physiology has improved.