Twin Sisters

This situation is not the same as the common form of fraternal twinning, in which two genetically different ova are fertilized by two genetically different sperm. In this case, the ova are genetically identical.
• Types
There are two mechanisms by which this might happen:
- Polar twins (or "polar body twins"), where two sperm fertilize an ovum, one of the two fertilizing a polar body; or where an ovum splits into identical copies, one containing a polar body, prior to fertilization, allowing it to be fertilized by two different sperm.