Siblings that are of different sexes are non identical. Twins of the same sex can start to have different physical features as they grow. Growth patterns (such as hair and eye colour, ear shape, teeth formation, hand and feet appearance) can indicate whether or not the children are identical. By around 2-years-old, their zygosity may be quite clear from their physical features.

Reasons why zygosity twin determination would be required:

It can be interesting to know whether multiple siblings are identical

• It may have been unclear in utero. Identical twins don’t always share the same placenta and sac: it all depends on when the egg splits
• Parents often want to know this information, and sometimes the twins themselves like to know when they’re older
• It can help reinforce the need to treat the siblings as individuals

It can be useful to know whether multiple siblings are identical

• It can help assess the likelihood of conceiving twins again: there is an increased risk for women who have had non-identical twins
• Some illnesses may affect monozygotic children so you would know whether both/all children need testing for a particular condition
• You may need this information to participate in any research about multiple births

In addition, some people buy a twin DNA test as a unique gift idea for multiple birth parents or their children and the DNA certificate can be used as a special keepsake or even wall art.