Common Health Questions

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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.

Further information

Summary

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.