Sometimes, two dominant follicles develop eggs and ovulate. If both are fertilized and subsequently implanted in the uterus, two embryos develop: twins. Because they developed from separate eggs that were fertilized by different sperm, they are called fraternal twins. Fraternal twins do not share any more genetic information than siblings born separately.

In contrast, the two daughter cells that remain after a fertilized egg undergoes its first division may separate and divide independently of each other. When this happens, they remain loosely connected while in the Fallopian tube, and the two blastocysts implant together in the uterine wall. They develop into two separate embryos. Because these embryos came from the same fertilized egg, they share identical genetic material and are called identical twins.


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“Boy or Girl” Not a just Metter of Chance