A somewhat technical explanation of Mono/Di twin pregnancy

2 babies??

2 babies??

I am now in the third trimester of my second pregnancy; this time with spontaneous, identical twins!

There is a lot of confusion out there about the different types of twins. Most people know that there are identical and fraternal twins, but unless you have a reason to know all the twin lingo, you may not be familiar with terms like monochorionic, dizygotic, monoamniotic, etc.

Here is a cool visual of the options:

twin image explanation

My twins are monozygotic, meaning they split from the same egg. So they are identical!

“So, basically, you’re the same person?”

We know this already, even before birth, because they share one placenta instead of having their own. That is what monochorionic means. (All babies sharing a placenta are identical, although sometimes they appear to share one placenta on ultrasound and then after delivery it is discovered that it was two placentas that fused, meaning there is a chance they are fraternal.) The diamniotic part means that they have separate amniotic sacs. The very thin membrane separating them is a very good thing, as it eliminates the risk of umbilical cords strangling each other or tying in knots, cutting off blood flow from the placenta. These two factors together tell us that the fertilized egg split between days 4-8, because if they split any earlier they would have their own placentas and sacs (dichorionic/diamniotic), and if they split later they would share one sac in addition to the placenta (mono/mono). Or, the latest egg splitting of all (between 13-15 days of conception) would result in conjoined twins.

Di/Di is the most common type of twin pregnancy, and many medical professionals will say these are fraternal twins. (Fraternal twins are dizygotic, or the result of 2 separate eggs being fertilized by 2 separate sperm. Basically, the DNA equivalent of full siblings that happen to be born at the same time.) There is still a 30% chance that Di/Di twins are monozygotic (or identical) twins that split very early on in the pregnancy, and this can be determined with a DNA test. If your di/di twins have the same blood type and are often mistaken for each other, it might be worth the $100 to send in a saliva swab. I’ve heard that other twin moms have used these companies to test: here, or here.

Mono/Di (sometimes called modi) twins do come with risks that Di/Di twins do not. Because of the shared placenta, they have a 10-15% risk of developing TTTS, or Twin-To Twin-Transfusion Syndrome. This is a potentially life-threatening condition, which, if severe enough, can be treated with laser surgery or delivery, if the pregnancy is far enough along that the babies would be safer out of utero. The risk is purportedly highest between weeks 16-24. In pregnancies with this problem (again, only possible when multiple fetuses share a placenta) one baby is receiving too much blood from the placenta, and one is not receiving enough. This is bad for both babies, as the recipient can develop heart problems from the stress of trying to deal with too much blood, and the donor twin can decline from not receiving the nutrients he or she needs. This is one of the main reasons mono/di twins are considered high risk.

In addition to regular appointments with an obstetrician, the mother of mono/di twins should be referred to an MFM, or maternal fetal medicine specialist. I have ultrasounds with my MFM every 2 weeks. They do a quick measurement of the amniotic fluid, and check to make sure both babies have visible bladders. If these two things look good, they don’t worry about TTTS and my 3 minute ($500 after insurance) ultrasound is over.

It is also recommended that mono/di twins are delivered early to avoid a deteriorating placenta. Some doctors want to deliver before 36 weeks, but my MFM would like me to go to 37 weeks if possible. Every doctor seems to have a different opinion on this, but you may want to reconsider your health provider if they aren’t taking the risks of your pregnancy seriously.

“And what I want is proper care for my high-risk pregnancy.”

Every 4 weeks, the MFM also measures the babies’ growth. This is a slightly longer ultrasound where they measure a femur from each baby, the head circumference, and the belly. The computer makes a weight estimate and growth percentile (comparing them to other babies of the same gestation) from these measurements.

My babies are measuring close in size and on the small side, about the 15th-20th percentile. They have told me they only begin to worry if they are under the 10th percentile.

There are a few more conditions that they are monitoring for. Unlike IUGR, or intrauterine growth restriction, which can be a complication in any pregnancy, sIUGR, or selective intrauterine growth restriction, only affects pregnancies with multiple fetuses. It is estimated to affect 10% of mono/di pregnancies. sIUGR can be caused by poor cord insertion, such as one cord being off to the side of the placenta, and the other directly in the middle. So one twin gets more than their share of the nutrients. The resulting size disparagement can cause an initial false diagnosis of twin to twin transfusion syndrome. Laser surgery is not as helpful with sIUGR as it is in cases of TTTS.


Welp, that’s what I’ve learned about the different types of twin pregnancies!

If you are early on in a Mono/Di pregnancy, try to eat a lot of protein and drink a lot of water, expect lots of appointments and ultrasounds, but try not to worry too much!


The odds are actually in your favor for a complication free pregnancy.


I don’t plan to get as technical in future posts (unless the need arises) so I thought I would get it all out of the way in case anyone else has questions or finds this stuff as interesting as I do! I hope you’ve enjoyed.