Twin pregnancy occurs when a woman carries two fetuses simultaneously. There are two main ways this can happen:
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Fraternal (Dizygotic) Twins: This is the more common type. Fraternal twins result when two separate eggs are fertilized by two separate sperm. Because they develop from two different eggs and two different sperm, fraternal twins are genetically distinct, just like any other siblings. They may be the same sex or different sexes.
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Identical (Monozygotic) Twins: Identical twins occur when one fertilized egg splits into two separate embryos. These twins share almost the exact same genetic makeup, and they are always the same sex. The reasons why an egg splits are still largely unknown, but it's generally considered a random event.
In summary, twin pregnancies are primarily caused by either the fertilization of two separate eggs or the splitting of one fertilized egg.
