Fertilization

Pregnancy occurs when a sperm penetrates an egg. The single sperm that first burrows into the egg is the successful one of millions of others contained in the man's semen. This is called fertilization, and usually takes place in the woman's fallopian tube after the matured egg has been released from one of her ovaries (ovulation).

Sperms may contain an X chromosome, or a Y chromosome. If an X fertilizes, the result will be a girl; if a Y penetrates, the result will be a boy. Sperms and eggs each contain 23 chromosomes, and the total number required of 46 chromosomes is achieved when a sperm and egg fuse at fertilization.

The fertilized egg, known as an ovum, immediately begins to divide into a growing cluster of cells. Between 5-7 days after ovulation, the fertilized egg implants into the wall of the womb (uterus) 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, carbon dioxide, amino acids, fats, vitamins and minerals from the mother's blood. It also allows the transfer of waste substances from the growing baby.

Embryonic and fetal 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 specialized 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 fetus. 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 (which normally breaks when the baby is ready to be born, releasing the amniotic fluid (the liquid that surrounds and 'cushions', 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.

The length of a normal pregnancy varies between about 37 and 42 weeks, although the delivery (term) date is calculated at 40 weeks from the first day of your last period. Only around 5% of babies are born on their due date. Babies born before 37 weeks are considered premature (or pre-term).