Twins account for 90% of multiple births but there is still so much misinformation among the public about twin pregnancies. People often think there are only two typical types of twins: Identical and Fraternal. There are actually 7 twin types but 3 common, and only 2 allow you to know if they are identical or fraternal prior to birth.
Monoamniotic/Monochorionic (mo/mo)
Mo/Mo twins share a sac and a placenta. These twins are always identical. Mo/Mo occurs when the egg splits 8-13 days after fertilization. This is also the most concerning type of twin in terms of risk factor, although many women do still carry close to, or to term.
Monochorionic/Diamniotic (mo/di)
Mo/Di twins share a placenta but have their own amniotic sacs. This set of twins is created when the egg splits 3-8 days after fertilization and are also always identical. Confusion over this fact does occur because Mo/Di is often misdiagnosed. When a Di/Di pregnancy occurs it is possible for the placentas to fuse and look like 1 on ultrasound. If there is no prior knowledge of 2 placentas existing it will be misdiagnosed as Mo/Di. The risks of mo/di are slightly lower than mo/mo but still a higher risk of preemie birth than singletons.
Diamniotic/Dichorionic (di/di)
This is the most common type of twin (76%). It means there are 2 sacs and 2 placentas. This is often stated as being fraternal solely because of the chorionicity but that is an error. There are 2 ways di/di twins can occur. The first is if 2 eggs are fertilized, and this would result in fraternal twins. However, di/di pregnancy can be identical if the egg split 1-3 days after fertilization. This happens roughly 30% of the time, and 20% of identical twins are Di/Di. Unfortunately, doctors don’t often relay this and will tell women they are having fraternal twins, leaving women shocked and confused later on when their twins are identical. There’s 3 ways to know if they are fraternal or identical. First is if they are boy/girl. Second is if they look completely different (if hair or eye colour is different this is usually an easy giveaway as those are genetically set). The Third is with a DNA test. The DNA test costs anywhere from $100-200 on average.
Di/Di have the best outcomes for twins and are usually carried full term, although many doctors push for early term delivery. The only major concern with di/di over a singleton pregnancy is for the mother and that is hemorrhaging due to the extra placenta, but this is still quite rare. Placenta privea can occur but it seems to be very marginally more common than singleton pregnancies.
Why is this information so important as a twin mom?
Knowing the type of twins you are carrying is critical to your risk factor and what kind of care and supervision your pregnancy will require, especially as you get towards the end of your second trimester into your third trimester. For the first half of your pregnancy your ultrasound and appointment schedule will look no different than a singleton pregnancy if no other signs of issues are seen. Every 4 weeks for an appointment, 1 scan at 20 week (likely you had an earlier dating scan that diagnosed twin pregnancy already).
Twins are often caught on ultrasound image kissing in utero.
If you are having mo/mo or mo/di twins you will likely be monitored much more closely to keep an eye out for TTTS (Twin to twin transfusion syndrome). This is when, because of both umbilical cords being attached to the same placenta, the placenta’s function is not shared evenly and one twin receives fewer nutrients. For this reason you’ll probably see a specialist as well as have bi-weekly ultrasounds through the last half of pregnancy. For Mo/Mo you will likely be monitored even more closely to avoid issues with cord entanglement. With Di/Di twins the bi-weekly ultrasounds probably won’t start until 28-30 weeks because the risks are much lower than with other twin types and TTTS does not occur in Di/Di twins.
Super fun disclaimer time: I am not a doctor, I am a twin mom who’s done loads of research but my information is simply a start. I encourage all twin moms to do their own research, which can be extremely frustrating, as I found out, because there is so little research done that looks at each twin type separately. Find a care provider you trust and just ask a lot of questions.
By: Nikki Waring
