What was my probability of twin having fraternal twins?

Identical twins and fraternal twins have little to do with each other at conception. Identical (also called monozygotic - literally "one zygote") twins happen when a single fertilized egg splits in two and develops into two separate foetuses. No one knows precisely why this happens or what the "risk" factors are. Fraternal twins (dizygotic - two zygotes), on the other hand, are two separate eggs released at the same time and fertilized by different sperm. There can be genetic factors, environmental factors, hormonal factors and physical predispositions, hence why fraternal twins seem to run in some families. If you were a fraternal twin, I'd say it was probably a higher likelihood that it had something to do or was a predisposing factor to a fraternal twin pregnancy, but I think if you don't have a strong familial history of dizygotic twins it was purely coincidental and perhaps had more to do with factors other than family history. IVF has a very high rate of fraternal twins because ...  more