About 1 in 90 spontaneous human births (1.1%) results from a twin pregnancy.
The incidence of spontaneous twin pregnancies depends on the mother's ethnicity and is as follows:
- Certain African tribes: up to 1 in 20
- Black Americans: 1 in 40-60
- Norway, Denmark, Netherlands: 1 in 49
- White Americans/White Europeans: 1 in 70
- Mixed Asian: 1 in 125
- Japanese: 1 in 150
- Chinese: 1 in 250
With fertility treatments and IVF the number of twin births can increase significantly as high as 1 in 3 pregnancies. 1 in 50 people is a fraternal twin in the USA, and 1 in 150 is an identical twin. 1/3 of all twins born in the US are identical, but in Japan the figure is 2/3.
There are three different types of twins:
- Dizygotic Twins
- Monzozygotic Twins (dichorionic or monochorionic)
- Polar Body Twins
Dizygotic twin pregnancies are more likely when the following factors are present in the woman:
- You are on fertility drugs
- You are over 40
- You are of West African descent (especially Yoruba or Hausa)
- You are greater than average height and weight
- You had several previous pregnancies.
- You are yourself a dizygotic twin
- You have a family history of dizygotic twinning.
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 sacs and two placenta.
Monozygotic (MZ) twins
Monozygotic 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. Monozygotic twins are also known as identical or maternal twins.
- If the embryo splits within 2-3 days after fertilization then the twins are dichorionic-diamniotic.
- If the embryo splits between 3-8 days, then the twins are monochorionic-diamniotic twins.
- If the embryo splits between 8-13 days, then the twins are monochorionic-monoamniotic twins.
- If the embryo splits after 13 days, then the twins are monochorionic-monoamniotic conjoined 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.
Sharing of the Placenta in Twins
- All dizygotic twins have two separate placentas (sometimes fused but still separate) and are in different sacs (Di-Di twins)
- One third of MZ monozygotic twins have two separate placentas and sacs, similar to dizygotic twins (dichorionic or didi). With di-di MZ twins the fertilized egg has split within 2-3 days after fertilization.
- 2/3 of monozygotic twins share a placenta (monochorionic-diamniotic or mono-di). With mono-di MZ twins the fertilized egg has split within 3-8 days after fertilization.
- About 1% of twins will share their inner sac (monochorionic, monoamniotic or mono-mono). With mono-mono MZ twins the fertilized egg has split within 8-13 days after fertilization.
- In very rare cicrumstances, the fertilized eggs splits 13+ days after fertilization, and this results in conjoined twins, twins that are joined at certain body parts
Dichorionic versus Monochorionic Twins
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 than dichorionic twins. Twins that are within the same sace (monochorionic-monoamniotic) have the highest risks of all twins, especially if they are conjoined 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.
Pregnancy Outcomes of Monochorionic and Dichorionic Twins