Mom just sent me this year’s newsletter from the Mid-Atlantic Twin Registry. I was telling Laura and Steph Lamb that I didn’t know many of the facts about twins in the newsletter’s “Did you know?” section. Did you know these? I didn’t even know that there was such a thing as “Half-Identical Twins”.

Identical Twins

• Only 1/3rd of all twins are identical.
• Contrary to popular belief, no genetic proof exists that identical twins “run in the family.” No hereditary influence for twinning has yet been identified.
• May have one shared placenta, two separate placentas, tor two placentas fused into one.
• They share 100% of their genetic markers.
• Are always the same sex.
• Have the same blood type.
• Not caused by fertility treatments or maternal age.
• Can result in conjoined twins or mirror image twins. Mirror image twins are created when the fertilized egg splits quite late (around days 9-12); any later and twins can be conjoined.

Fraternal Twins

• Can be hereditary on mother’s side. The tendency to hyperovulate, or release more than one egg in a cycle, is a genetic trait that can be passed from mother to daughter.
• May have two separate placentas or two placentas fused into one.
• Share about 50% of their genetic markers, or the same as signleton siblings.
• May be same sex or male/female.
• May or may not have the same blood type.
• Can be attributed to fertility treatments or advanced maternal age.

Half-Identical Twins aka “Polar-Body Twins”

• It is not known what percentage of twins is half-identical.
• May have two separate placentas or two placentas fused into one.
• Share about 75% of their genetic markers, mor than fraternals, but less than identicals.
• May be same sex or male/female.
• May or may not have the same blood type.
• These twins occur when the mother’s egg splits before fertilization and then each half is then fertilized by a sperm. This results in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). However, DNA-based zygosity tests are currently not available to determine if twins are polar-body twins.

If you want to read the newsletter yourself, a PDF is available here.

Now that I’ve posted it, I can stop carrying around this newsletter to show you next time I see you.