Multiple pregnancies occur when either more than one egg is released for fertilisation, or when one egg is fertilised and subsequently splits.
In identical multiples, the egg splits shortly after fertilisation (within 2 weeks). Identical triplets and higher order multiples occur when one or more of these eggs splits again. Identicals will be of the same sex and have the same genetic make-up as one another.
In non-identical multiples, more than one egg is fertilised around the same time, and these are known as fraternal, rather than identical.
It is quite common for triplets to contain 2 identical twins and a fraternal triplet, two eggs fertilised, one of which splits, or completely fraternal ie 3 eggs released and fertilised. Recent changes to IVF rules mean that only 2 embryos are now transplanted back into the womb meaning the rates of fraternal triplets resulting from IVF are dropping.
There are certain factors which can affect the likelihood of twins, these can include :
Fertility treatment – drugs which stimulate the ovaries to produce more eggs can result in multiplesFamily history. It is thought that fraternal multiples can be hereditary through the female line of the family. This is because the releasing of more than one egg at one time is thought to be genetic. There seems to be less influence if twins are on the father’s side of the family. There is however growing evidence to support identicals as being more than just the fluke of nature that the medics would have us believe.
- Age. Statistics show that women in their late 30s or older are more likely to conceive twins.
- Previous maternities. The chances of having twins increases with the number of pregnancies you have.