Fertilisation

If your menstrual cycle is regular, as a general rule fertilisation happens about a week after you have finished menstruating or 14 days before your next period begins.

About seven to 10 days after this, the fertilised ovum is implanted in the lining of the womb. By the end of another week, it is firmly attached by its primitive placenta, which links the developing embryo to its mother.

The placenta is the organ through which foodstuffs and oxygen are carried from the mother to the baby and waste substance are carried form the baby to the mother. It is an absolutely crucial organ to the healthy progress of pregnancy because it produces pregnancy hormones that are responsible for maintaining the health of the developing baby, the uterus and the female genital organs. These hormones also prepare the woman's body for labour and for birth.

The ovum is usually fertilised about a third of the way along the Fallopian tube by a single sperm which was deposited with millions of others in the vagina after ejaculation. Within a few seconds of ejaculation the sperm become mobile with the lashings of their whip-like tails.

This then carries them at top speed out of the acid conditions of the vagina and through the neck of the cervix, which has become more fluid during ovulation, into the cavity of the uterus.

In a few seconds, the sperm pass through the uterus and enter the Fallopian tube to meet the ovum that is travelling down the tube towards them.

Sperm are chemically attracted to the comparatively enormous ovum and attach themselves to it like limpets over the whole surface. However only one sperm pierces the outer coat of the ovum. Instantly the egg loses its attraction, hardens its outer shell and all the superfluous sperm let go. This whole process, from ejaculation to fertilisation can take less than 60 minutes.

A ripe ovum usually survives for only 12 hours, and a maximum of 24. Sperm retain the power to fertilise for not much longer than 2 hours; 36 hours is probably the limit. Fertilisation is therefore unlikely unless sexual intercourse occurs one or two days before or immediately after ovulation.

Only the head of the sperm fuses with the ovum, forming a single cell. The cell divides into two in the first 24 hours: by the fourth day it is a ball of over 100 cells.

This ball of cells floats free for the first three days in the cavity of the uterus, nurtured on uterine 'milk' secreted by glands in the uterine wall.

By the end of the first week, it has implanted into the uterine lining, where it is continuously bathed in a lake of its mothers blood, allowing food and waste to pass to and fro. Until week 8 the developing baby is known as an embryo, after which it is called a foetus, Latin for 'young one'.