• Dr Peter Chew

Twin pregnancy: how does it occur?

The number of twin pregnancies has increased in recent years. This is due to the fact that women are having babies late and consequent fertility treatments using IVF procedures.There are two types of twins, identical (monozygotic) and non-identical (dizygotic). About a third of all twins are identical and the rest, non-identical.Identical twins occur when a single fertilised egg (zygote) divides in two, creating twins that share the same genes, same sex and almost similar appearance. The cause of identical twins is not well understood. All pregnant women have approximately the same chance of having identical twins – about 1 in 250. Identical twins do not run in families.Non-identical twins happen when two separate eggs are fertilised by two separate sperm.


Unlike identical twins, there are some factors that make having non-identical twins more likely:

• Ethnic groups: twinning rate is highest among Nigerians and lowest among Japanese


• Age: Older mothers are more likely to release more than one egg during ovulation.


• Family history: There is a hereditary tendency for twinning to occur in some families


• In vitro fertilisation (IVF): During the procedure, more than one embryo is usually transferred.Often, identical twins can be inferred in the womb, as they often share one single placenta. For nonidentical twins, it can be deduced if they are of different genders (a boy and a girl) or if they have different blood types. However, the only reliable way of identifying whether the twins are identical or not is by genetic DNA testing.


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