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Tundraco's Daily Living Guide to Pregnancy

The Three Types of Twins

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The Three Types of Twins
By Rochelle Caviness

Most people are aware that twins can be fraternal or identical. Current research indicates that there may be a third variety called Polar Body Twinning. Identical Twins
Identical twinning occurs when a single fertilized egg divides in half, and each half develops into an identical child. The twins develop into nearly identical individuals because the egg divides after fertilization, therefore each twin shares the same chromosomes. Fraternal Twins
Whereas Identical twins are both formed from the same egg, fraternal twins develop from two totally separate eggs. Although extremely rare, it is possible for fraternal twins to have different fathers if each male contributed sperm that fertilized a different egg. For this to occur, viable sperm from both donors must be present at the time of conception. Polar Body Twins
Polar body twinning is believed to occur when an egg divides before fertilization, and thereafter, each half is then fertilized with different sperm. This gives rise to a state identified as half-identical because half the chromosomes that each of the babies received was identical.
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