...but what do FRATERNAL and IDENTICAL TWINS explain:
Fraternal twins are the result when two different eggs (ova) are fertilized by two different sperm. This leads to the development of two separate placentas, each with its own chorion and amnion. These are more common than identical twins and account for about 2/3 of twin pregnancies.
Identical twins develop when a fertilized egg splits. Depending on when the split occurs will determine if the twins share a placenta, with either one or two chorions and amnions, or if they each develop their own placentas. In general, the later the spit occurs, the more likely that the twins will share one placenta.
Fraternal vs. Identical Twinning
Even after they are born, it is sometimes difficult to know whether twins are identical or fraternal. It can be easier if they:
- share one placenta (identical)
- are different sexes (fraternal)
- have different blood types (fraternal)
It is harder to know if they are the same sex, have the same blood type, or if there are two placentas, since they could then be either fraternal or identical twins.
Don't be fooled by a fused placenta (fraternal twins), which can look like it is just one placenta, or if the twins don't look alike. Sometimes, factors during the pregnancy, especially twin to twin transfusion syndrome, can lead to identical twins that have very different birth weights and are mistakenly thought to be fraternal. And sometimes, fraternal twins can look enough alike to be confused with identical twins.
If you or your doctors aren't sure of the zygosity of your twins, you can consider having DNA testing done.