A woman becomes pregnant when one of her eggs becomes fertilised by a sperm. This usually happens after sexual intercourse, which takes place when the penis is placed inside the vagina and the man has an orgasm and ejaculates sperm. The penis needs to be erect in order for this to happen.
At the height of sexual excitement, also called an orgasm, millions of sperm spurt out of the penis into the vagina. Sometimes, when a man has an ejaculation near the opening of a woman's vagina, the sperm can also swim up the vagina towards the uterus. A woman can fall pregnant whether she has an orgasm or not. Some of the sperm travel through the uterus to the fallopian tubes. Here sperm can survive for between five and seven days.
If there is an egg in one of these tubes, one of the sperm cells can fertilise it and form a single cell. (The egg is 85, 000 times larger than the sperm cell, but it doesn't really make a difference). There is only an egg in the fallopian tube if a woman has ovulated.
This single cell then attaches itself to the lining of the uterus more or less nine days after fertilisation – and the cycle of life begins all over again. Inside this tiny cell is the baby's genetic coding for life. It is only in the last few decades that scientists have begun to understand how this genetic coding works and what role chromosomes and DNA play in everyone's lives.
The uterus can stretch slowly to accommodate a fully-developed foetus, the term by which a baby is known before it is born. Birth takes place more or less nine months after fertilisation, usually through the vagina, which can stretch in all directions to let the baby out. This process is called labour and can last anything from an hour or two to a day. If the baby cannot be born through a woman's vagina, it can be removed directly from the uterus. This is called a Caesarean section.
(Health24, updated June 2010)