About 1 in 90 spontaneous human births (1.1%) results from a twin pregnancy.
There are three different kind of twins:
Dizygotic twins are twins which result from the fertilization of 2 different eggs with 2 different sperms. Dizygotic twin pairs can be girl/girl, boy/boy, or girls/boy. Other words for dizygotic twins are also fraternal or non-identical twins. The rate of spontaneous dizygotic twins varies by population. It is highest among African-Americans and lowest for Asians.
Dizygotic twin pregnancies are more likely when the following factors are present in the woman:
Infertility treatments increase the rate of dizygotic twins, with about 35% of pregnancies from IVF being twin pregnancies. All dizygotic twins have separate placentas.
All dizygotic twins are dichorionic, which means they have two separate out sacs and two placenta.
Monozygotic (MZ) twins result from the fertilization of one egg and one sperm. The fertilized embryo then splits within days after fertilization resulting in two individuals which usually share the same chromosomes. Monozygtic twins are also known as identical or maternal twins.
The rate of monozygotic twins remains at about 1 in 333 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly, though with IVF the rate of monozygotic twins is also increased.
One third of MZ twins have a separate placenta (dichorionic or didi), and 2/3 of monozygotic twins share a placenta (monochorionic-diamniotic or monodi). About 1% of twins will share their inner sac (monochorionic, monoamniotic or monomono).
When identifying twins, it is important to find out whether they are dichorionic or monochorionic. Twins that share a placenta (monochorionic) are at increased risk of certain complications such as twin-to-twin transfusion syndrome.Monochorionic twins must be watched in pregnancy much closer thatn dichorionic twins.
Polar body twins happen very rarely, and they results from one egg fertilized by two different sperm. Polar body twinning would result in "half-identical" twins.
Whether twins are MZ or DZ can be determined by analysis of DNA samples from blood or other tissues. All unlike sex twins are DZ. All monochorionic twins are MZ. Thus DNA analysis is required for the approximate 50% of twins who are like sex with dichorionic placentas.