It is useful to understand how eggs and sperm are normally formed, and how conception occurs to understand the causes of infertility and how they are targeted in ART.
The hormones which control the production of sperm and eggs are called gonadotrophins. There are two types of gonadotrophins: follicle-stimulating hormone (FSA) and luteinising hormone (LH). In men, they stimulate the testicles to produce sperm and testosterone. In women, they act on the ovaries, where the eggs develop. The female sex hormones, oestrogen and progesterone, are produced by the ovaries when eggs mature and are released (ovulation).
The production of sex hormones and the release of an egg is known as the menstrual cycle. It is counted from the first day of the period until the day before the start of the next period. In an average cycle of 28 days, ovulation happens on day 14. However, cycle length varies between women, and it is important to note that ovulation occurs earlier in women with shorter cycles and later in women with longer cycles.
Sperm are produced at the rate of about 300 million per day. They take some 80 days to mature. Each sperm has a head, which contains the genetic material, and a tail, which propels it up through the vagina, uterus, and Fallopian tubes where the egg is fertilised.
Conception occurs when an egg and a sperm come together. At ovulation, an egg is released from the ovary into the fallopian tube. If sperm is present at that time, the egg can be fertilised. The fertilised egg then starts to divide and becomes an embryo. After ovulation, the ovary produces progesterone which prepares the lining of the uterus - the endometrium - for the growing embryo. If an embryo does not form or attach to the endometrium (implantation), the level of progesterone drops and the next period starts.
A few days after implantation, the embryo starts to produce Human Chorionic Gonadotrophins (HCG) - the hormone that gives a positive pregnancy test reading.
Women are born with all the eggs they will ever have. As a woman ages, her eggs age with her, diminishing in quantity and quality.