At the beginning of each menstrual cycle the pituitary gland in the brain releases a hormone which stimulates the ovaries to produce follicles. One of these follicles grows faster than others to become the 'dominant follicle'. It is from this follicle that an egg will be released each month at ovulation.
During this monthly cycle the ovaries produce several hormones, most importantly oestrogen, which promotes growth of the follicles, and progesterone, released after ovulation, which prepares the uterus for pregnancy.
After ovulation, the egg passes through the fallopian tube to meet one of several sperm cells swimming towards it. Fertilisation occurs when a sperm cell penetrates the outer layer of the egg. The fertilised egg continues to the uterus and pregnancy begins when it implants in the lining (known as the endometrium). If the egg is not fertilised, or the embryo does not progress, the endometrium is shed as a menstrual period approximately 14 days after ovulation.
Almost all causes of male infertility are usually related to sperm disorders - either too few in number or of inadequate quality. It's for this reason that one of the first investigations of the infertile couple is a semen analysis laboratory test. The male, unlike the female, is constantly renewing his supply of germ cells, and a single ejaculate of semen may contain as many as 200 million individual sperm cells. A sperm count of 20 million per millilitre semen is considered normal for fertilisation. The sample should also show that more than half the sperm cells are motile, and around one-third of normal shape.
It is not always possible to determine an exact cause, but it is now clear that many cases of female infertility today are simply age-related and explained by nothing more than a reduction in the number and quality of eggs available in the ovaries. It's for this reason - what is known as reduced 'ovarian reserve' - that fertility rates begin to decline after the age of 35 (in both spontaneous and assisted conception).
Today, as more and more women defer their first pregnancy until careers and finances are more settled, we are seeing ever more cases of infertility whose cause is related to the availability of fewer good quality eggs and embryos. And this too is one of the big challenges of fertility treatment today.