- Dizygotic twins (fraternal/non-identical) are twins from two different eggs and two different sperms.
- Monozygotic twins (maternal/identical) are twins which split from one egg and one sperm after fertilization.
How Dizygotic Twins are Formed
Dizygotic twins form when two separate sperms fertilize two separate eggs. Each fetus lies within it's own two membranes (chorion and amnion) and they do not share blood vessels.
How Monozygotic Twins are Formed
Monozygotic (identical) twins form when one sperm fertilizes one egg. The fertilized egg then splits later on. Depending on when the fertilized egg splits, you can have either different sacs (dichorinic/diamniotic), the same outer sac and two inner sacs (monochorionic/diamniotic), or they are within the same 2 sacs (monochorionic/monoamniotic).
- Split Before 3-4 days After Fertilization: When the fertilized egg divides before 3-4 days after fertilizatiion 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.
- Split beetween 3-8 days after fertilization: The cells divide at between 3 and 8 days and they are monochorionic/diamniotic.
- Split beetween 8-13 days after fertilization: The cells divide around between 8 and 13 days are are in one sac monochorionic/monoamniotic (dangerous because cords can become tangled).
- Split after 13 days after fertilization: If the division happens after day 13, they are all in the same sacs and conjoined twins happen
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 dichronionic/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 and it is similar in all ethnic groups and does not vary with maternal age or parity. However, the incidence of monozygotic twins may be 2-3 times higher following in-vitro fertilization procedures, possibly because with these methods the architecture of the zona pellucida is altered although the mechanism remains uncertain.
Prevalence of Dizygotic Twins
The prevalence of dizygotic twins varies with:
- Ethnic group (up to 5-times higher in certain parts of Africa and half as high in parts of Asia)
- Maternal age (2% at 35 years)
- Parity (2% after four pregnancies)
- 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.
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.
Entangled umbilical cords of monochorionic-monozygotic twins, both in the same sac: