About 1 in 90 spontaneous human births (1.1%) result from a twin pregnancy. Twins account for about 1% of all spontaneous pregnancies, but up to 35% of IVF pregnancies are twins. 

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 United States are identical, but in Japan, the figure rises to 2/3.

There are different types of twins:

1. Dizygotic Twins
2. Monozygotic Twins (dichorionic or monochorionic)
3. Polar Body Twins

Dizygotic (Fraternal) Twins

Dizygotic are twins which result from the fertilization of two different eggs with two different sperms. Dizygotic twins form when two separate sperms fertilize two separate eggs. They are genetically like siblings, sharing about 50% of the genes.  Each fetus lies within its own two membranes (chorion and amnion) and they do not share blood vessels. All dizygotic twins have two membranes each (dichorionic-dizygotic) and two placentas, though occasionally the placentas attach. Dizygotic twin pairs can be girl/girl, boy/boy, or girls/boy. Dizygotic or fraternal twins are non-identical. 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:

• 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 also 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 placentae.

Identical (Monozygotic) 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. They are genetically identical sharing about 100% of the genes. Depending on when the fertilized egg splits, you can have either different sacs (dichorionic/diamniotic), the same outer sac and two inner sacs (monochorionic/diamniotic), or they are within the same 2 sacs (monochorionic/monoamniotic).

1. Split within 3-4 days after fertilization: When the fertilized egg divides before 3-4 days after fertilization then the twins are dichorionic/diamniotic. That is the membrane configuration as if the twins were dizygotic except that monozygotic twins have the same genetic composition (DNA), while dizygotic twins share only 50% of DNA.
2. Split between 3-8 days after fertilization: If the cells divide between 3 and 8 days they are monochorionic/diamniotic. This can be dangerous because they share a placenta and blood vessels.  
3. Split between 8-13 days after fertilization: When the cells divide between 8 and 13 days, they are in one sac monochorionic/monoamniotic (dangerous because cords can become entangled).
4. Split after 13 days after fertilization: If the division happens after day 13, they are all in the same sacs and conjoined twins can happen.

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 has also increased.  

Polar Body Twins

Polar body twins happen very rarely, and they result from one egg fertilized by two different sperm. Polar body twinning would result in "half-identical" twins.

Incidence

In the last 20 years, the incidence rate of twins has increased. This increase is most marked for dizygotic twins. It is estimated that about one-third of the increase in multiple births is a consequence of childbearing later in life, but the majority of the effect is due to the use of assisted reproduction techniques (ART) like ovulation induction and in-vitro fertilization (IVF).

• 100% of dizygotic twins are dichorionic/diamniotic
• About 30-40% of monozygotic twins (and 100% of dizygotic twins) are dichorionic/diamniotic
• About 60-70% of monozygotic twins are monochorionic/diamniotic
• About 1-2% of monozygotic twins are monochorionic/monoamniotic

Prevalence of Monozygotic Twins

The prevalence of monozygotic (maternal/identical) twins is about 1 in 250 pregnancies, is similar in all ethnic groups, and does not vary with maternal age. However, the incidence of monozygotic twins may be 2-3 times higher following in-vitro fertilization procedures.

Prevalence of Dizygotic Twins

The prevalence of dizygotic twins varies with:

1. Ethnic group (up to 5-times higher in certain parts of Africa and half as high in parts of Asia)
2. Maternal age (2% at 35 years)
3. Parity (2% after four pregnancies)
4. Method of conception (20% with ovulation induction)

In general, about two-thirds of all spontaneous twins are dizygotic (fraternal/non-identical) and one-third are monozygotic (maternal/identical).

The incidence of twins (mostly dizygotic) increases significantly if the pregnancy is achieved with ovulation induction or it is an IVF pregnancy, with some centers reporting a 35-40% incidence of twins in IVF patients. 

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). 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 circumstances, the fertilized eggs split 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 space (monochorionic-monoamniotic) have the highest risks of all twins, especially if they are conjoined twins. 

Zygosity
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.

Monochorionic vs. Dichorionic Twins

• Twins, in general, are at increased risk for certain pregnancy complications including premature birth, death, and other anomalies.
• When monozygotic twins split later than 3-4 days after fertilization, they are called 'monochorionic' twins and they have complications which are many-fold higher than those of dichorionic twins. 
• Monochorionic twins are at an especially high risk of complications and death because they often share the placenta and have one outer membrane (chorion) as compared with dichorionic twins which have a separate membrane (dichorionic). 
• Monochorionic twins have a significantly higher risk of complications such as twin-to-twin-transfusion-syndrome (TTTS) and require more intensive surveillance during pregnancy. 
• Many doctors recommend delivery of monochorionic twins around 34-36 weeks of the pregnancy to decrease the risk of complications.