Being the mom of twins, I have always been in the midst of remarks and questions, but the most common misnomer of them all is that they are look-alike or identical! Well, there are several types of twins!
1. On Basis of Zygotes:
• Identical (Monozygotic): Well the most popular of the lot, Identical twins usually look pretty similar, sometimes to the extent that parents have to identify birth marks to differentiate between their duo. A single egg cell if fertilized by single sperm cell, which later on divides itself in two different babies, generating the same DNA profile of them both.
• Fraternal (Di –zygotic): The second most common set of twins is fraternal, since they look different from each other. Ideally, a set of fraternal twins are like any two siblings, their faces can be an exact match or a complete opposite. So if you see a pair of twins with the same face, don’t always believe that they are identical!
• Co-joined Twins: Co - joined twins are identical twins, where the formed zygote tries to split in the womb, but does not slit completely. This leads to a pair of twins which walks around as one, being joined at one part of the other.
2. On the Basis of Placentation
• MO/MO: These are the least common types of twins and the most risky among all twin pregnancies. Monochorionic/Monoamniotic means that both twins are in the same sac and have the same placenta.
• DI/DI: These are the most common types of twins. Dichorionic/Diamniotic means that each twin has their own placenta and their own Amniotic sac. This is known to be the least complicated of all twin pregnancies.
• MO/DI: Monochorionic/Diamniotic twins means that these twins share a placenta and are in the same outer sac, but have a thin membrane dividing them, creating separate amniotic sacs for both twins. They are considered mid risk as a twin pregnancy.
Based on other parameters:
3. Mirror Image Twins: Mirror Image twins seem to be mirror images of one another. If one is left handed, the other will be right handed. If one’s hair swirls to left, the other’s hair will swirl to the right. Mirror Image twins have no determination test per se, and are identified by observation only.
4. Super fecundation: If a woman has produced two eggs, and both these eggs get fertilized by a different set of sperms (yeah, that’s quite possible) then the resulting two fetuses would be born out of super fecundation. Definitely, these are twins that are half brothers/Sisters!! They can even be different races.
5. Semi Identical Twins: This happens in a very rare case, when one egg is fertilized by two sperms, which later on splits, forming semi identical twins. Again, they may or may not look similar at all!
6. Polar Twins: In this situation, one egg splits into two parts and is then fertilized by two different sperms. Polar twins look almost similar, but their DNA is very very different.
7. Twins born on different dates: This can very well happen!! In this case, one of the twins needs to be brought out in case of an emergency, while the other baby is delivered at a later time. This difference in birthdays can be in months!
8. Identical Twins with different Genders: The common myth is that if they are a boy and girl twin, they have to be fraternal! In a rarest of rare cases, one male zygote splits into two but one of them undergoes a genetic mutation, dropping the Y chromosome. One of them then develops into a male baby and the other one into a female baby.
Feeling wiser now? Share it with your friends too, especially to parents of twins you know!
Source for banner image: rialeephotography.com