Degree of Separation
The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week.
In very rare cases, twins become conjoined twins. Furthermore, there can be various degrees of shared environment of twins in the womb, potentially leading to pregnancy complications.
It is a common misconception that two placentas means twins are dizygotic. But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.
| Type | Description | Day |
|---|---|---|
| Dichorionic-Diamniotic | Normally, twins have two separate (di- being a numerical prefix for two) chorions and amniotic sacs, termed Dichorionic-Diamniotic or "DiDi". It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts ), in 99.7% of all pregnancies, and in 18–36% (or around 25%) of monozygotic (identical) twins.
DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons. |
Dichorionic-Diamniotic twins form when splitting takes place by the third day after fertilization. |
| Monochorionic-Diamniotic | Monochorionic twins share the same placenta.
Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins. Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused. |
Days 4-8 |
| Monochorionic-Monoamniotic | When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization
Sometimes, monochorionic twins also share the same amnion. This situation occurs in 1–2% of monozygotic twin pregnancies. Monoamniotic twins are always monozygotic. The survival rate for monoamniotic twins is somewhere between 50% to 60%. Consequently, if twins are monoamniotic that means that the two babies will be sharing a placenta and as a result, due to the small capacity of sharing a sac, the umbilical cord has an increased chance of being tangled around the babies. Because of this, there is an increased chance that the newborns may be miscarried or suffer from cerebral palsy due to the lack of oxygen. |
Monoamniotic twins occur when the split takes place after the ninth day after fertilization. |
| Conjoined twins |
When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. |
If the division of the zygote occurs later than the 12 days then conjoined twins are usually the result. |
Read more about this topic: Twin
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