So you think you want a baby

Day One: The first day of your menstrual period (when bleeding starts) is day one of your menstrual cycle. Your ovaries produce varying quantities of the female hormones oestrogen and progesterone, but at the very start of your cycle your ovaries aren't producing very much.

The hypothalamus (a small area in the brain behind the eyes) detects that your oestrogen levels are getting low, and so the hypothalamus sends a chemical signal to the pituitary gland (a pea-sized structure at the base of the brain). This signal causes release of a hormone that stimulates your ovaries, which causes them to start to turn a follicle (an immature egg) into a mature egg. This fertility hormone is called follicle-stimulating hormone (FSH).

During each cycle your ovaries respond to FSH by starting to enlarge anywhere from eight to 15 immature eggs. The egg that matures first then starts to make hormones of its own. These mix with the other hormones that are circulating, and this probably causes the other immature eggs to stop growing. In any one cycle, it's usually only the dominant follicle which matures completely and is released at ovulation, while the rest die. If more than one egg matures and is fertilised, it results in twins or triplets.

During the time that the egg is maturing, before ovulation, the lining of the uterus (womb) is thickening, nourished by circulating oestrogen. After ovulation, cells in the corpus luteum (egg sac) produce progesterone. Progesterone changes the thickened lining of the womb so that it is ready to be shed when you start your period.

It takes an egg around two weeks to completely mature. When the egg is almost fully matured, it starts to make more oestrogen and other hormones, which acts as a signal to the pituitary gland to release a large amount of its next fertility hormone, luteinizing hormone (LH).

Day 14 - This surge of LH then tells the ovary to release the egg, and this process is called ovulation. Within 24 to 48 hours, the follicle releases its egg and the egg makes its way into one of the two fallopian tubes. Ovulation usually happens around 14 days before the next period is due, and a newly ovulated egg can be fertilised by a sperm at any time in the next few days - potentially up to seven days after ovulation. This, obviously, is the best time for you to conceive.

If the egg is not fertilised, your period will begin as soon as the egg sac deteriorates and your progesterone levels start to drop off. On this day, your ovaries aren't very active. The hypothalamus, of course, detects that the level of oestrogen is low, and this begins the whole cycle all over again...

If you're trying to conceive, you need to work out when your next period is due, and then count back 12 to 16 days from this date. This will give you a range of days when you will probably be ovulating. If you have a regular 28-day cycle, the 14th day is often said to be the best day to get pregnant. This method only works if you have relatively regular cycles.