Mechanics - Conception
Fertilisation is the word used to describe the joining of a human egg and sperm. When a man ejaculates into a woman’s vagina, the ejaculate contains millions of sperm. The sperm move up the vagina, through the cervix, into the uterus and into the Fallopian tubes. If an egg has been released and is in the Fallopian tube one of these sperm may reach the membrane of the egg and may penetrate it.
The moment a sperm penetrates the membrane of the egg, a chemical process occurs which makes the outside of the egg hard and stops other sperm from entering it. At this point, the 23 chromosomes from the woman and the 23 chromosomes from the man are joined to give the baby its genetic makeup. So, all babies get half their genes from their mother and half from their father.
As the fertilised egg moves towards the uterus – and, it will take the egg three to five days to reach the uterus - cell division begins. When the egg has reached the uterus it will plant itself into the rich lining of the uterus where it will grow and develop into a foetus.
The fertilised egg has a 52% chance of growing into a male and a 48% chance of growing into a female.
Most women only have one baby at a time but sometimes there might be more than one baby. There are two types of twins (two babies) – one is called fraternal and the other is called identical.
Fraternal twins are where two eggs have been released and they both get fertilised by two different sperm. These twins are not identical and may be two girls, two boys or a boy and a girl.
Identical twins happen when one egg that has been fertilised splits in two. Identical twins are always the same sex. Sometimes we see identical twins that may be joined together – these are call conjoined twins. This is very rare but happens when the fertilised egg has not completely divided in half.
Triplets can also occur when separate eggs are fertilised or when a fertilised egg splits into three.