As a twin mom, I have learned a thing or two about twins. Most people know there are fraternal and identical twins. Since becoming a twin mom, I have learned there are more terms than these to describe different types of twins. Read on to learn about each type of twin, and why some fraternal twins are actually identical!
How Twins are Formed
Before we dive into the different types of twins, it’s important to have an understanding of how twins are formed to begin with.
Identical twins are formed when a fertilized egg splits into two separate eggs. This is referred to as monozygotic twins, since they split from one zygote. These types of twins have identical DNA and as such, are always the same sex.
Fraternal twins, on the other hand, form from two separate eggs that are fertilized by two separate sperm. They are referred to as dizygotic twins because they form from two separate zygotes. Fraternal twins are as similar in DNA as non-twin siblings. However, since they both grow and develop in one uterus, they are still referred to as twins.
Related – Signs of Twin Pregnancy – How to Know if it’s Twins!
Different Types of Identical Twins
Even though identical twins are always formed from one zygote, they can develop differently in the womb. There are several things that can happen once the egg has been fertilized.
Di/Di (dichorionic/diamniotic) Twins
If the fertilized egg splits before implanting in the uterus – two separate sacs and two separate placentas will form. These twins develop similarly to fraternal twins, except they are identical because they were formed from one egg and one sperm.
Mo/Di (monochorionic/diamniotic) Twins
If the fertilized egg impants in the uterus and THEN splits, you will have twins with a shared placenta but two separate gestational sacs.
Mo/Mo (monochorionic/monoamniotic) Twins
Lastly, if the implanted embryo splits a bit later on, you can have twins with one placenta and one sac. This is considered the most risky type of twin pregnancy.
Types of Fraternal Twins
Fraternal twins always form from two separate eggs. This can happen when more than one egg is released during a woman’s cycle, and the eggs are both fertilized by separate sperm. It also happens often with In Vitro Fertilization, if more than one embryo is transferred into the womb. This is how I ended up with my fraternal twins!
Fraternal twins can only develop one way in the uterus. Each embryo develops in it’s own amniotic sac, and has it’s own separate placenta. Therefore, all fraternal twins are Di/Di (dichorionic/diamniotic) twins. This is the safest scenario for a twin pregnancy.
How can Fraternal Twins be Identical?
Sometimes parents of fraternal twins will notice that their twins look much more alike than regular siblings. This sometimes leads to the parents having DNA tests done to determine if their twins are actually identical. But how can this be?
It’s actually pretty simple. If your fraternal twins were not the result of IVF (in-vitro fertilization) and you have no way of knowing if they were formed from one or two eggs, there is a small chance they could be identical twins. As long as they are the same sex, there’s really no way to tell for sure without a DNA test. Even though all Mo/Di and Mo/Mo twins are identical, Di/Di twins can be either fraternal or identical.
Risks of Different Types of Twins
Interestingly, the type of twins you are carrying can determine how risky the pregnancy will be for the babies. Certain types of twins are riskier than others.
While twin pregnancy in itself is more risky than a singleton pregnancy, Di/Di twin pregnancy presents the least risk to the twins. This is the safest scenario for twins to develop.
Mo/Di twins, on the other hand, are sharing a placenta. This can allow for complications such as twin-to-twin transfusion syndrome (TTTS) or twin anemia-polycythemia sequence (TAPS) to develop.
The riskiest type of twin pregnancy is found with Mo/Mo twins. Since they share a placenta, they are also at risk for twin-to-twin transfusion syndrome (TTTS) or twin anemia-polycythemia sequence (TAPS). However, since they are also in the same amniotic sac, there is the additional risk for cord entanglement.
Other Types of Twins
Believe it or not, there are still a few more types of twins!
Conjoined twins – this type of twin is the result of monozygotic twins where the zygote doesn’t fully separate.
Superfetation twins – rarely, a woman can release an egg even after she is already pregnant. If this egg is fertilized and implants successfully, this type of twin pregnancy results. If the mother has sex with multiple partners, there is even the chance that the twins have different fathers!
Mirror Image Twins – this is a type of identical twin that can happen when the embryo splits later on. These twins can have features such as birthmarks that are mirror images of each other.
Polar Body Twins – Very rarely, an egg can split before being fertilized. If both halves of the original egg are then fertilized by separate sperm, these twins will have identical DNA from their mother. This makes them basically “half-identical” twins.
Hopefully this helped you to understand the differences in types of twins! Please sign up for our email list for all things twins, parenting, relationships and more!
