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Conception is the process that begins with the fertilisation of an egg and ends with the implantation of this egg into the woman's womb. A woman is most likely to conceive just after she ovulates, when an egg has been released from one of her ovaries (halfway between her monthly periods). During sex, sperm are ejaculated from a man’s penis into the woman’s vagina. One ejaculation can contain more than 300 million sperm.

Most of the sperm leak out of the vagina again, but some begin to swim up through the cervix. When a woman is ovulating, the mucus in the cervix is thinner than usual to let the sperm pass through more easily. Sperm swim into the womb and into the fallopian tube. Fertilisation takes place if a sperm joins with an egg and fertilises it in the fallopian tube.

During the week after fertilisation, the fertilised egg (which is now an embryo) moves slowly down the fallopian tube and into the womb. It's already growing. The embryo attaches itself firmly to the specially thickened womb lining. This is called implantation. Hormones released by the embryo and the ovaries prevent the womb from shedding. This is why women miss their period when they're pregnant.

Development of the egg
Five to seven days after ovulation (the release of an egg from the ovary), the fertilised egg implants itself into the wall of the womb and produces root-like outgrowths called villi. These help to anchor it to the lining of the womb and will eventually grow into the placenta (the organ that feeds and protects the baby until birth).

The placenta maintains and nourishes the baby by enabling the transfer of oxygen, amino acids, fats, vitamins and minerals from the mother's blood. It also allows the transfer of waste substances from the baby.

Embryonic and foetal stages
From the time of implantation into the wall of the womb until approximately the eighth week of life, the developing baby is known as an embryo. Development is rapid during this stage, as specialised cells begin to form the vital organs, nervous system, bones, muscles and blood.

After the eighth week of pregnancy, the developing baby is called a foetus. It is about 2.4cm (1 inch) long, with most of the internal organs formed. External features, such as the eyes, nose, mouth and ears, can be seen and fingers and toes start to appear.

As the unborn baby grows, so does the womb. A fluid-filled double membrane surrounds the baby. This normally breaks when the baby is ready to be born, releasing the amniotic fluid (the liquid that surrounds the baby).

During pregnancy, the baby floats freely in the amniotic fluid and constantly swallows this fluid, excreting it in their urine. Much information about the baby's health can be obtained during a procedure called amniocentesis, where a small sample of amniotic fluid is taken for testing. However, amniocentesis is quite invasive (involves going into the body) and carries a small risk of miscarriage, so it is usually only offered to women when there is a significant risk that their baby will develop a serious condition or abnormality.