8 Types of Twins You Didn’t Know Existed

A lot of people know about fraternal twins and identical twins, but few know about the eight different types of twins you didn’t know existed until now. Check out this comprehensive list of the various types of twins and what makes them unique!

Double Identical (Dizygotic)

In dizygotic twins, also known as fraternal twins, two eggs are released and fertilized by two different sperm. These are what most people think of when they imagine twins; like any siblings, dizygotic twins share roughly half their DNA. These types of twins include identical twins (also called monozygotic) and fraternal twins.

Fraternal (Dizygotic)

Fraternal types of twins are just like any other siblings. They’re genetically different from one another. This means that, in fraternal twins, each egg has been fertilized by separate sperm. Essentially, fraternal twins come about when two eggs released during ovulation happen to be fertilized by two different sperm.

Half-Identical (Monozygotic)

These are your classic identical types of twins. They come from a single zygote that splits in two and develops into two embryos. This type is represented by famous sets like Mary-Kate and Ashley Olsen. If you’ve seen them side-by-side, you probably wouldn’t be able to tell them apart (hence their career). They share 100% of their DNA with each other, just like any other siblings.

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Same Sex (Monozygotic)

Identical twins occur when a single sperm fertilizes an egg, creating one zygote that later splits and forms two embryos. In most cases, same-sex twins are always identical, but in rare cases, one twin may be male and the other female. About 1 out of every 65,000 births results in a set of same-sex twins. While their chromosomes come from only one pair of ancestors, these children still have all four sets.

Mirror Image (Monozygotic)

Also known as identical twins, mirror-image twins occur when one zygote (fertilized egg) splits into two parts and develops into two embryos that are genetically identical. This type refers to as a monozygotic twin pregnancy.

Polar Body Twins

Sometimes, one egg splits in two and then each egg gives rise to a baby. The twins are considered ‘polar body twins.’ This rare type of twinning is very different from identical or monozygotic (MZ) twins. Despite being genetically identical, MZ twins look just like any other siblings born at different times—they don’t have any special physical characteristics that set them apart. Polar body twins, on the other hand, can be hard to tell apart—especially when they’re infants.

Gonadal Identity Ambiguous

This type of twinning occurs when a fertilized egg divides into two and becomes ovotesticular, which means that it has both testicular and ovarian tissue. Gonadal Identity Ambiguous (Diploid Pseudohermaphroditism) is very rare and only affects 1 in every 125,000 live births.

Ovarian Torsion – Ovarian Cyst Embryo

When an embryo implants itself into a woman’s fallopian tube, it grows and expands until it bursts through its protective sac (the zona pellucida). The embryo then cuts off from its nutrient supply and begins to die. Since in most cases these embryos have no chance of survival, they usually reabsorb back into a woman’s body or expells with her menstrual cycle.

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