This is a question I get a lot lately from those familiar with carrying multiples. Had you asked me this question 4 months ago, I would have assumed the only answer was fraternal vs. identical twins. While those are the two major classifications for twins, there are actually various “types of twins” dependent upon if the pregnancy is a result of two embryos implanting, or one embryo splitting and implanting. Bear in mind, I am by no means an expert in this area and most of this information I have collected through my MFM visits and twins forums/boards.
Identical vs. Fraternal Twins
Identical vs. fraternal twins refers to zygosity, or one egg/one sperm vs. two eggs/two sperm.
Identical twins come from a single egg that has been fertilized by one sperm, and then splits into two separate embryos early in development. Both of those embryos implant and develop into a positive pregnancy test. They will have the same DNA, and look exactly alike, because they came from ONE embryo. Identical twins will always be the same sex – either boy/boy or girl/girl.
Fraternal twins develop when two eggs are fertilized by two separate sperms, and both implant. Fraternal twins can look a lot alike, but will often be easier to tell apart than identical twins. Their DNA is different. There can be boy/boy, girl/girl, or boy/girl sets.
Di/di, mo/di, mo/mo, & conjoined twins
Zygosity (identical vs. fraternal twins), really has little impact on the treatment and prognosis of the twin pregnancy. What needs to be considered is chorionicity and amnionicity, or the number of placentas and the number of amniotic sacs that are present.
Chorionic = Placenta
Amniotic = Amniotic Sac
The image below can be helpful along with the following explanations of “types” of twins.
Dichorionic/Diamniotic (di/di) Twins
Two separate placentas and two separate amniotic sacs (one for each baby). These are the most common type of twins and have the lowest associated risk. Fraternal twins are di/di. Some identical twins can be di/di if the embryo splits VERY early.
Monochorionic/Diamniotic (mo/di) Twins – what we are having!
Mo/di twins are identical and share a placenta but each baby has their own amniotic sac. These twins are more risky than di/di because of the shared placenta. A shared placenta is just that – shared. Sharing does not translate to equal. Mo/di twins are at risk for twin-to-twin transfusion syndrome (TTTS).This occurs when the blood vessels in the shared placenta are connected. This can result in one baby receiving more blood flow (the recipient) and the other receives too little blood (the donor). In the past, TTTS often had devastating results, but with proper monitoring and medical intervention, twins with TTTS now have a better chance of survival. TTTS occurs in approximately 15% of identical twins. More about TTTS will be covered in my next blog!
Monochorionic/Monoamniotic (mo/mo) Twins)
The least common and most risky, mo/mo twins share a placenta AND an amniotic sac. They are always identical. Mo/mo twins are at risk for TTTS and are also at risk for issues resulting from sharing the same amniotic sac. Because both babies are in the same sac, there is a chance their umbilical cords may become entangles or compressed, which can lead to one or both babies not receiving enough nutrients. Moms of mo/mo twins often end up in the hospital during the later second half of their pregnancy for bed rest and inpatient monitoring (multiple ultrasounds a week) to ensure the cords are not becoming tangled. At our first ultrasound we thought we were having mo/mo twins, but it was later confirmed that there was a thin membrane separating the amniotic sac into two.
Conjoined twins are also identical, and occur when the egg does not completely separate into two embryos in the womb. They can share organs, tissue and/or limbs. Conjoined twins also share a placenta and an amniotic sac. I am not sure what their risks are for TTTS or cord entanglement.
I am currently carrying, identical, mo/di twin girls. Because of the risks associated with mo/di twins, I see a Maternal Fetal Medicine (MFM) specialist for regular ultrasounds, along with my OB. Having twins is scary, and having mo/di twins is even scarier. We are trying to take things one day at a time and continue to pray for a healthy pregnancy and delivery.