Congratulations on your pregnancy. There are two main types of twins: monozygotic and dizygotic. Monozygotic twins are commonly called identical twins, and dizygotic twins are fraternal twins.
Identical twins are formed when a single fertilized egg splits during development. Therefore, monozygotic (identical) twins have the same genetic makeup.
Fraternal twins, on the other hand, result when two separate eggs are fertilized and implanted into the uterus. Dizygotic (fraternal) twins may look similar but do not share the same genetic makeup.
It can be difficult to know whether you are having identical or fraternal twins, but there a few clues. Identical twins are the same sex. So if you are having a boy and a girl, it’s safe to assume they’re fraternal.
If your twins are sharing a single placenta, they are most likely identical; however, if there are two placentas, they could be either identical or fraternal.
Once your babies are born, they may have their blood types done in the nursery. (Blood types are typically performed on infants of mothers who have type O or negative blood.) If your babies have different blood types, they are fraternal twins.
If your babies are born and are the same sex and share the same blood type, there is still a chance they may be fraternal twins. Medically, there is no reason parents need to know the answer to this question; however many parents are curious.
For this reason, there are commercial labs that offer mail-in blood tests. Insurance does not pay for the screening since it is elective. Most areas have local support groups for twin parents, such as Charlotte Mothers of Multiples. It would be a good idea to investigate any laboratory by speaking with other moms, as well as your healthcare provider prior to sending them money and blood specimens.