Introduction to Formation of twins

Twins are rare and special, and they happen in about 1% of all pregnancies. 30% are identical and 70% are fraternal twins. The egg is fertilized by a single sperm cell and forms a blastocyst after a week. Identical twins start from a single fertilized cell. But the egg splits into two separate embryos sometime in the first week after conception. Identical twins have identical DNA. In fraternal twins, there are two eggs released and fertilized by two sperm cells. They develop separately and have different DNA.

If the animation above isn't working, click here to get the Shockwave player.