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TWINS: EXPLAINED

Twins are two individuals who form in the same uterus. There are a few different kinds:

1. FRATERNAL TWINS (non-identical)

These account for two-thirds of twins.

Two eggs are fertilized by two sperm and both implanted into the walls of the uterus at the same time.

These eggs form separately, in separate sacs adjacent to each other.

They share 50 percent of their DNA, as they would with their other non-twin siblings.

They can be different genders, though most (around 60 percent) are same-sex twins.

Research suggests fraternal twins run in families, typically down the maternal line.

2. IDENTICAL TWINS

A single egg is fertilized by one sperm and embeds in the uterus wall.

But at some point, it splits from one zygote into two.

They do not always share the same amniotic sac, but they always share the same placenta.

Those that share the same amniotic sac are known as monoamniotic, while those that have their own are diamniotic.

Most identical twins split within eight days of conception.

Those that split between eight and 12 days are known as mono-mono twins - incredibly rare. 

Those that split after 12 days tend to be conjoined.

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