How multiples are made

Sometimes a twin or triplet pregnancy just happens. Other times, specific factors are at play. For example, a twin pregnancy is more likely as you get older. That's because hormonal changes can cause an ovary to release more than one egg at a time. Use of fertility treatments such as in vitro fertilization to become pregnant also raises the odds of twins or other multiples.

Fraternal twins — the most common kind of twins — form when two different sperm fertilize two separate eggs. The twins grow and develop inside two separate fluid-filled sacs called amniotic and chorionic sacs. And they receive oxygen and nutrients through two separate pregnancy organs in the womb called placentas. Fraternal twins can be of the same sex or different sexes.

Identical twins form when a single fertilized egg splits early in pregnancy and develops into two fetuses. Identical twins might share a placenta and an amniotic and chorionic sac. Or the twins might share a placenta and each have separate sacs. Both have the same genetic material. They'll be the same sex and share the same traits such as blood type, hair color and eye color. Rarely, identical twins are born with connected body parts or organs. These are known as conjoined twins.

In a pregnancy of three or more, the babies may be identical, fraternal or a mix of both.