All of a woman's eggs will already be present when she is born. From the early years of childhood, she starts to lose eggs, but it is calculated that a woman aged 50 still has about 1000 of her eggs left.

A woman's cycle begins with a hormonal signal from the pituitary gland at the base of the brain. It releases a follicle stimulating hormone (FSH), which stimulates the ovaries. FSH stimulates a group of about 20 follicles on the surface of the ovary to grow. Within the follicles are the developing eggs.

Another hormone, oestrogen, is produced by the ovary and in response to increasing oestrogen levels, the largest follicle continues to develop. This is why most pregnancies result in only a single baby.

Another hormone, luteinizing hormone (LH), also produced by the pituitary gland, causes the follicle in the ovary to release the egg. This release of the egg (or ovum) is called ovulation.

After ovulation, the empty follicle forms a structure called the corpus luteum which produces a hormone called progesterone. Levels of progesterone rise after ovulation and prepare the womb to receive a fertilized egg (embryo). Fertilization by a sperm, if this occurs, takes place in the Fallopian tube, which then moves the early embryo along towards the womb itself. If the egg is not fertilized, or the embryo does not implant in the womb, the progesterone levels fall and a period starts. The whole cycle then begins again.

2. The man
The man's role in conception is the production and ejaculation of sperm cells. To fertilise an egg, a man must be capable of producing adequate numbers of swimming (motile) sperm. Only a few of these sperm cells will in the end reach the egg, and only one will actually enter and fertilise it.

During puberty, the testes become active and start to produce sperm. From his teens until about the age of 70, a man will typically produce 5,000 sperm cells every minute. Ejaculation produces semen, which is a mixture of two per cent sperm cells and 98 per cent liquid produced by the glands in the testes. Production of sperm is under the control of FSH and LH, the same hormones that control the woman's fertility.

When are the chances of fertilization and conception greatest?
Fertilization only happens if intercourse takes place almost around the time of ovulation. Ovulation only takes place once in every menstrual cycle, which means that there are only a few days each month when a woman can fall pregnant. However, intercourse and ovulation don't need to be at exactly the same time as healthy sperm can survive for about 48 to 72 hours inside a woman's body and a woman's egg lives for about 12 to 24 hours after ovulation.

Fertility is therefore a result of sperm and egg meeting each other almost immediately after ovulation. Ovulation occurs 14 days before the onset of a period. This is reasonably predictable if the cycle is regular, but may be very unpredictable if the cycle is irregular.

What does fertility depend upon?
The hormone balance must be correct to ensure an egg develops and ovulates normally. Intercourse must take place during the fertile part of the cycle. There must be an adequate number of motile sperm and no mechanical barrier such as blocked Fallopian tubes, which may prevent sperm reaching the egg.

Infertility investigations
Infertility investigations will vary according to the couple's particular problem. It is sensible to ensure immunity to rubella (German Measles) before attempting to become pregnant. This involves a simple blood test. Also, it is advisable that the woman is taking a daily dose of folic acid to prevent spina bifida in her baby if she falls pregnant.

Other investigations are described below.