Under normal circumstances, men are fertile all the time. Sperm are produced continuously by the testes at a rate of around a hundred million per day. The testes begin sperm production at puberty and continue until the man dies, although the number and quality decline with age.
Women are fertile for only one day each cycle, at ovulation; that is, when an ovum (egg) is released from one of her two ovaries. A woman is born with some two million immature ova in her ovaries. Most of these ova never fully mature but gradually disintegrate throughout the woman ’s reproductive
By the time a woman develops a mature reproductive system following puberty, approximately 400,000 ova remain in her ovaries. Only 400-500 of these ova will be stimulated to full maturity by the woman’s hormones and about once a month, one will rupture from either the left or right ovary at ovulation. This ovum will live for a mere 12 – 24 hours after which it dies if it has not been fertilised by a sperm cell. The woman continues to release approximately one ovum per month until menopause.
Sperm ejaculated into the vagina will normally die in its acidic environment in a few hours or less. In the days preceding ovulation however, a woman’s cervix (which is at the base of the uterus) produces specialised fluid which can nourish and sustain the sperm for up to five days. This fluid is also important for the filtering of abnormal sperm and the transportation of healthy sperm into the uterus (Fig. 1). Of the several hundred million sperm deposited into the vagina during ejaculation, only around 200 will reach the ovum, and only one will be able to fertilise it. Once ovulation has occurred, the specialised fluid is no longer required and the woman’s hormones cause the cervical fluid to change and her resting body temperature to rise about 0.2°C. If the ovum is fertilised, the newly formed embryo begins to develop and will implant in the soft nutritious lining of the woman’s uterus, reaching birth some 8½ months later.
If there is no fertilisation, the woman’s uterus sheds the lining it had prepared for an embryo in what is known as menstruation or a ‘period’.