Final answer:
When a woman ovulates and both oocytes are fertilized, fraternal twins result, also known as dizygotic twins. These twins are no more identical than siblings born at different times and will have individual placentas. Identical, or monozygotic twins, come from a single fertilized egg that splits into two embryos.
Step-by-step explanation:
If a woman ovulates two oocytes and both are fertilized, fraternal twins result. This condition is known as dizygotic twinning. In this scenario, the twins develop from two separate eggs fertilized by two separate sperm and are genetically as similar as siblings born at different times. These twins will have individual placentas and are no more identical than any other set of siblings.
By contrast, identical twins, or monozygotic twins, occur when a single fertilized egg (zygote) splits into two separate embryos. This can occur at various stages in early development. If this split happens at the two-cell stage, the resulting monozygotic twins will have individual placentas. However, if the division occurs at the blastocyst stage, they will likely share a placenta and chorionic cavity.