Even before a baby girl is born she will have around 1-2 million eggs stored in her newly-formed ovaries. After birth her body will not produce any more eggs and throughout her fertile years, the ovaries will release a total of only three to four hundred eggs.
A girl's first period usually occurs between the ages of ten and fourteen. From then on, until the menopause she will have a monthly period at the start of each menstrual cycle.
Ovulation; when an egg is released into the fallopian tube ready to be fertilised by a sperm occurs 12-14 days before a period, so if a woman has a 28 day cycle this is halfway between periods, around day 14. If the cycle is regularly 30 days she will normally ovulate between day 16 and day 18 of her cycle, taking day one as the first day of her period.
It is also quite normal to have shorter or longer cycles which last anything between 23 and 35 days.
Physical signs of ovulation include an increase in vaginal discharge, which changes from white and creamy to clear and slippery, breast tenderness, bloating, mild abdominal discomfort, a slight increase in body temperature and an increased sex drive.
Light blood spotting can also occur at the time of ovulation in some women.
Although the ovary prepares up to 20 eggs each month for release, only one (or two if it's going to be non-identical twins) prevails and makes it out of the ovary to the fallopian tubes. These tubes will gently transport the tiny egg on its journey to the uterus (womb) using a rippling wave-like motion; the journey from ovary to uterus takes about five days.
In simple terms, if the egg becomes fertilised by a sperm at this mid-cycle stage it will settle into the uterus lining (endometrium) and begin to grow into a baby. If it is not fertilised, the egg will be rejected by the body, along with some of the endometrium and will appear as the next period.
It is a well known fact and the butt of many jokes that hormones control a woman's menstrual cycle.
There are five main hormones involved and the whole process begins in the brain. At the beginning of the cycle, gonadotrophin-releasing hormone (GnRh) starts everything off and instructs follicle stimulating hormone (FSH) to stimulate the ovaries to produce another hormone; oestrogen.
Oestrogen then encourages around 20 eggs to ripen, one of which will mature quicker than the rest and that will be the one that is released. Oestrogen also causes the lining of the uterus to thicken, in preparation for pregnancy (should fertilisation occur).
Normally the cervix (the neck of the uterus) produces a thick, opaque mucus that sperm cannot penetrate. However, just before ovulation oestrogen changes this mucus so that it becomes thin and clear. This change allows the sperm to swim through the cervix into the uterus and up to the fallopian tubes where fertilisation may take place.
Soon after this, another hormone; progesterone alters the mucus in the cervix again so that it becomes impenetrable to sperm once more. Progesterone also acts on the lining of the uterus to prepare it for a fertilised egg.
Ingeniously, as soon as fertilisation occurs, the woman’s pituitary gland will stop producing FSH so that no more eggs can mature during the pregnancy.
If fertilisation doesn't occur or the egg doesn't implant into the uterus, the oestrogen and progesterone levels will drop and the womb lining will begin to produce chemicals that reduce the blood supply to the uterus and cause it to contract.
This is then the beginning of the woman’s next menstrual cycle.