Most twins are fraternal; each develops from a separate egg and sperm. The ovaries usually release one egg each month to be fertilized, but occasionally two or more eggs may be released. Fraternal twins each have their own placenta and amniotic sac. (Sometimes these twins will be described as dizygotic, meaning two zygotes, or two fertilized eggs.)
Because each twin develops from the union at a different egg and a different sperm, these twins look no more alike than any brother or sister do. The twins can be both boys, both girls, or one of each.
Sometimes, for unknown reasons, one fertilized egg splits early in pregnancy and develops into two or more fetuses. Two fetuses formed this way are identical (or monozygotic) twins. They share a placenta, but each usually has its own amniotic sac. Because they share the same genetic material at the beginning, they are the same sex and have the same blood type, hair color, and eye color. These twins can look so much alike that even their mothers may have difficulty telling them apart.
Approximately 2% of births in the US are multiple pregnancy. Most involve twins; triplets occur in 1 of 7,600 pregnancies. Three or more fetuses can be formed by more than one egg being fertilized, a single fertilized egg splitting, or a combination of both.
Multiple pregnancies are found before delivery. It may be suspected if:
- Fraternal twins tend to run in your family
- Your uterus grows more quickly or is larger than expected.
- More than one heartbeat can be heard.
- You have been taking fertility drugs.
- You have extreme bouts of nausea and vomiting in the first trimester.
- You feel more fetal movement than you did in any pregnancies you had before.