Types of Twins
Twins are more complex than just “identical” and “fraternal”!
Fraternal (or Sororal) Twins
When two eggs are released by the mother, and both are fertilized and implant successfully, two completely different gene sets develop into a fetus. The two eggs almost always develop their own placenta, and their own amniotic sac. These are scientifically known as dizygotic (di - two, zygote - egg) twins, and there’s a genetic basis to whether a female is predisposed to have them.
Identical (Monozygotic) Twins
When only one egg is released and fertilized, there’s a chance of identical twins forming. Unlike fraternal twins, there’s no genetic basis for the formation of identical twins, so it’s uncommon for families to have multiple sets of them. There are several types of monozygotic twins:
Dichorionic-Diamniotic Twins (DiDi) These twins have two (di-) chorions (which means they also have two placentas) and two amniotic sacs. They occur when splitting of the fertilized egg (embryo) occurs less than 72 hours after fertilization. Fraternal twins are also considered “DiDi”. Around 25% of identical twins are DiDi. These twins have the lowest mortality rate, at about 9%.
Monochorionic-Diamniotic (MoDi) These twins have one (mono-) chorion (meaning one placenta), but two amniotic sacs. This occurs when the embryo splits between 4 and 8 days after fertilization. Because they only have one placenta, there’s a risk of twin-to-twin transfusion syndrome. Between 60-70% of identical twins (and about 0.3% of all pregnancies) are MoDi.
Monochorionic-Monoamniotic (MoMo) "MoMo" twins share both their chorion and amniotic sac. They make up about 1-2% of all monozygotic pregnancies, and occur when the embryo splits between 9-12 days after fertilization. They have the highest mortality rate of all monozygotic twin pregnancies, with only 50-60% surviving to birth. In addition to twin-to-twin transfusion syndrome, the risk of one or both fetuses becoming entangled in the umbilical cords and cut off from nutrition can lead to significant disability even when both babies survive.
Conjoined Twins When twins separate later than 12 days after fertilization, the split is very rarely complete, and conjoined twins occur. In general, the later the split, the less complete the split is. They have the highest mortality rate among twin pregnancies.
"Mirror Twins"
When MoMo twins split between the 10th and 12th day after fertilization, they often develop into “mirror twins" - that is, they’re the inverse of one another. Things like dominant handedness (left vs right-handedness), dominant eye, dental development, and even the direction of their organs inside their body can be reversed. Not all MoMo twins that split during this time period are “mirror”, but many are.
Illustrations:
The Practice of Obstetrics. Edited by Charles Jewett, 1901.
See also:
Conjoined female twins in the Nuremberg Chronicles. By Hartmann Schedel, 1440-1514.
Placentation diagram by Kevin Dufendach at Wikipedia
