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?.
