A: Dear Bitsy,

Congratulations on your special pregnancy! Statistically, you are more likely to be carrying fraternal twins: two thirds of twins are fraternal, while one third are identical.

Fraternal twins develop when two separate eggs are fertilized by separate sperm. Each baby has its own amniotic sac and its own placenta, and genetically they are no more similar than any other siblings in a family.

In contrast, identical twins share all of the same genetic material. They will look alike and are the same sex. Identical twins begin when one egg is fertilized by one sperm. At some point during early development, the embryo splits into two identical embryos. The timing of this split can vary. Depending on how early in the pregnancy the embryos separate, the twins may grow (1) within the same amniotic sac and share the same placenta (called monoamniotic, monochorionic), (2) within different amniotic sacs yet share the same placenta (diamniotic, monochorionic), or (3) have different sacs and different placentas (diamniotic, diamniotic), similar to fraternal twins.

Ultrasound exams can provide clues about whether some twins are identical or fraternal. If the ultrasound clearly shows that your twins are growing within the same amniotic sac (monoamniotic, monochorionic), they must be identical. (This situation is not very common, however, and is considered high risk because the two babies' umbilical cords could entangle.)

If your ultrasound has revealed that you are carrying babies of each sex, this is conclusive evidence that the twins are fraternal! However, if both twins are the same sex and each has a separate amniotic sac, it's usually quite hard to determine identical versus fraternal status prior to delivery.