Types of Twins Compared
Not all twins are made the same way. This page compares every recognised type of twin in one table, then explains how doctors tell them apart.
Each type has a full story elsewhere on Actual Twin, from mirror twins to the rare semi-identical kind.
How fraternal and identical twins begin: National Human Genome Research Institute, Public domain
Every type of twin at a glance
| Type | How they form | DNA shared | Same sex? | How common |
|---|---|---|---|---|
| Identical (monozygotic) | One egg, one sperm; the embryo splits | Almost 100 percent | Yes, almost always | About 3 to 4 per 1,000 births |
| Fraternal (dizygotic) | Two eggs, two sperm | About 50 percent, like siblings | Either | Varies a lot by country and family |
| Semi-identical (sesquizygotic) | One egg, two sperm | All of mother's, part of father's | Either | Only 2 confirmed cases |
| Mirror-image | Identical twins from a late split | Almost 100 percent | Yes | Roughly a quarter of identical twins |
| Conjoined | Identical twins whose split is incomplete | Almost 100 percent | Yes | Very rare |
How doctors tell them apart
During pregnancy, ultrasound scans look at the placentas and membranes. Twins who share one placenta are almost always identical. Twins with two placentas can be either kind, because identical twins that split very early also develop separate placentas.
After birth, the surest answer comes from a DNA test, which compares genetic markers from a cheek swab. Many families do this simply out of curiosity, and it can matter for medical reasons such as organ donation.
Boy-girl twins
Boy-girl twins are almost always fraternal, because identical twins come from one egg and one sperm and so share the same sex chromosomes. The rare exceptions include the semi-identical twins confirmed in Australia in 2019.
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Facts on this page were checked against these sources (updated Oct 12, 2026). Spot an error? Email hello@fireempireshop.com.