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Zygosity Chart for Multiples

Monozygotic Twins Dizygotic Twins Polar Body Twins
Form when a single fertilized egg splits into two. Form when two eggs are fertilized by two separate sperm. Occur when a single unfertilzed egg splits into two and is fertilized by separate sperm.
Also called identical. Also called fraternal. Sometimes referred to as hybrid or half-identical twinning.
Only one-third of all twins are identical. Two-thirds of twins are fraternal. It is not known what percentage of twins falls in this third category.
Don't "run in families" except by coincidence. No hereditary influence for identical twinning has been identified. Can be hereditary on the mother's side. The tendency to hyperovulate (release more than one egg in a cycle) is a genetic trait that can be passed from mother to daughter. Not known.
May have one shared placenta, two separate placentas, or two placentas fused into one. See examples. May have two separate placentas or two placentas fused into one. See examples. May have two separate placentas or two placentas fused into one. See examples.
Share 100% of their genetic markers. Share about 50% of their genetic markers, same as singleton siblings. Share about 75% of their genetic markers, more than fraternals but less than identicals.
Are always same sex. May be same sex or male/female. May be same sex or male/female.
Have the same blood type. May have the same blood type or different. May have the same blood type or different.
Not caused by fertility treatments, birth control pills or maternal age. No one knows what causes identical twinning. Can be attributed to fertility treatments, advanced maternal age, birth control pills or other factors that influence twinning. No one knows what causes this type of twinning to occur.
May be contained in one sac in utero. Develop separate sacs in utero. Develop separate sacs in utero.
Can result in conjoined twins or mirror image twins. Not conjoined. Not conjoined.
May be at risk for Twin-to-Twin Transfusion Syndrome (TTTS) Rarely at risk for TTTS. Rarely at risk for TTTS.

Can you tell by looking? Take the quiz: Twin Type Quiz: Identical or Fraternal?.

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