NHS website - Do twins run in families?

Do twins run in families?

It depends on the type of twins.

There's no evidence that identical twins run in families. However, non-identical twins can run in families. A couple is more likely to have twins if there are twins in the woman's family.

Identical twins

Identical twins happen when 1 fertilised embryo splits into 2. Identical twins are sometimes called "monozygotic" twins.

Each baby will have the same genes. Identical twins are therefore the same sex – both boys or both girls – and they look very alike.

Non-identical twins

Non-identical twins happen when 2 eggs are fertilised by 2 sperm at the same time. Non-identical twins are sometimes called "fraternal" or "dizygotic" twins.

Some women may be genetically more likely to produce more than 1 egg during a menstrual cycle (hyperovulation). This makes it more likely they will have twins.

There are also non-genetic factors that can make non-identical twins more likely, including:

  • ethnic group – twins are more common in some ethnic groups than others
  • the age of the mother – the older the mother, the higher the chance of twins
  • whether the woman has already had children
  • infertility treatment

Non-identical twins do not have the same genes. They may be the same sex – both boys or both girls – or a boy and a girl. They will probably look no more alike than any other brothers and sisters.

Read the answers to more questions about pregnancy.

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