So for fraternal twins the woman just releases two eggs at the same time. It’s uncommon to do that but not sure rare. Then two separate sperm fertilize the two separate eggs.
For Identical twins the feralization happens normally, one egg one sperm. But very early in development when the blastocyst splits in two. A blastocyst is the clump of undifferentiated cells that will develop into an embryo and then fetus then baby. Those two split blastocysts will then develop separately into two twins. IF they don’t split completely that’s when you get conjoined twins.
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