Do twins always run in families?

Content supplied by NHS Choices

Twins can sometimes run in families. However, it depends on the type of twins. For example, whether the twins are:

  • non-identical (fraternal or dizygotic), or
  • identical (monozygotic).

In the UK, two thirds of all twins are non-identical and one third are identical.

Non-identical twins

Non-identical twins come from two different embryos. Two separate eggs are fertilised by different sperm, and two embryos develop.

Non-identical twins tend to run in families.

If you’re female, your chance of having non-identical twins may be higher if, for example:

  • you’re over the age of 30,
  • you’ve been pregnant before,
  • you’ve had a previous multiple pregnancy or birth, or
  • there’s a history of non-identical twins in your mother’s family.

Women who’ve had non-identical twins before have a higher risk of conceiving non-identical twins in a later pregnancy.

Women in some ethnic groups are more likely to have non-identical twins. For example:

  • women from African ethnic groups are more likely to have non-identical twins than white women, and
  • women from Asian ethnic groups are less likely to have non-identical twins.

Identical twins

Identical twins happen when a fertilised embryo splits into two, soon after a sperm fertilises the egg.

Identical twins don’t usually run in families. And the risk factors outlined above do not affect your chance of having identical twins.

The chance of having identical twins does not vary among different ethnic groups.


NHS Choices

Published Date 2010-12-15

Last Review Date 2008-08-12

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