Congratulations, you’re having twins! Likely one of the first things you said (after picking your jaw up off the floor) was, “Well, what KIND of twins am I having?” Everyone knows there are two types of twins: fraternal and identical.
Please always remember, none of the info here should ever replace discussions you have with your personal doctor. They are great talking points and should be used as such.
Us twin moms know it is much more complex than that, but here we’re going to focus on Monochorionic Diamniotic (MoDi, Mono Di or MCDA) twins, also known as identical twins, and the additional complications that could arise with this type of pregnancy, in order to better prepare you for the signs and symptoms you’ll need to spot if something seems wrong. Identical Twin Pregnancy
What kind of twin pregnancy do I have?
After the tech gives you the wonderful news that there are two babies in your belly the first thing they do is determine the chorionicity and amnionicity of your twins. In its most basic terms, this is how many placentas (chorion) and amniotic sacs your babies have. You should have this scan between weeks 11 and 13 along with a Down Syndrome screening called the nuchal translucency scan.
What does Mono Di twins mean?
Mono Di twins are monozygotic, meaning they come from one egg (mono = one) that split between days 3 and 7 after fertilization. They have one placenta/chorion and two amniotic sacs. If they split early enough they can develop two placentas (dichorionic diamniotic identicals) or if they split later they share everything (monochorionic monoamniotic, or MoMo).
True DiDi (fraternal) twins came from two eggs (dizygotic), have two separate placentas and amniotic sacs, and, as a result, they do not carry many of the same risks as Mono Di twins. Identical Twin Pregnancy
How common are mono di twins?
Mono Di twins occur in 0.3% of all pregnancies. 75% of monozygotic twin pregnancies are monochorionic; the remaining 25% are dichorionic diamniotic.
Are Mono Di twins high risk?
Almost all twin pregnancies are considered high risk and you will most likely be seen by the Maternal Fetal Medicine unit exclusively, or through periodic visits in conjunction with your regular obstetrician. Although the majority of twin pregnancies are uncomplicated, monochorionic twins are more likely than dichorionic twins to have more serious complications.
What is the difference between mono twins and identical twins?
Mono Mono twins (MoMo) and Mono Di twins (MoDi) are both considered identical twins.
Once you know what kind of twins you’re having it’s time to coordinate care. Sharing a placenta means both mom and babies require special care to ensure the healthiest and safest pregnancy.
First, you need a good obstetrician who understands the intricacies of a Mono Di twins pregnancy. You also need a Maternal Fetal Medicine Specialist (MFM) or a Perinatologist. My midwife who discovered the twins directed me to an MFM for special care, who in turn referred me to a great new OB (obstetrician).
MFMs and perinatologists fully understand the risks of a Mono Di twins pregnancy and are able to detect problems before they cause bigger issues. They are responsible for monitoring growth and fluid levels and performing in-depth ultrasounds at the most critical times of development. Identical Twin Pregnancy
While the majority of the time everyone is fine, there is too much at risk to treat a Mono Di twins pregnancy as one would a singleton or even a fraternal twin pregnancy. Technology and the medical community have come a long way in understanding the risks and with more and more professionals advocating for proper care and monitoring, carrying identical twins is no longer as scary as it was in the past.
The number one thing a specialist will do is give you an in-depth ultrasound every two weeks starting at 16 weeks. These scans will measure growth, amniotic fluid levels, anatomy, and blood flow. Sure, it seems like a lot – but think of all those sonograms you’ll have!
This is so important as almost everything that goes wrong when two or more babies share a placenta can be caught with an ultrasound and treated or monitored until the babies can be born safely (after 24 weeks). Make sure your OB not only okays this many scans but recommends it, otherwise you should find someone who does.
Twin To Twin Transfusion Syndrome
Likely the first thing you will hear about is Twin-to-Twin Transfusion Syndrome. This occurs in 15-20% of Mono Di twins pregnancies and happens when there is an abnormal share of blood vessels connected to the placenta. One twin, the recipient, receives too much blood flow and the other, the donor, receives too little.
This leads to an unequal distribution of amniotic fluid because the donor twin is dehydrated and not making enough urine. TTTS can progress quickly and may lead to fetal demise of one or both the twins if it is not detected early.
There is no way to prevent TTTS or to tell if you are at risk of developing it. While there can be symptoms — the mother may feel discomfort, or measure large — the only way to accurately diagnose TTTS is through an ultrasound. This is most important in weeks 16-24, when the babies are at the highest risk of developing TTTS, but it can develop at any time.
While TTTS is the most well-known complication in Mono Di twins pregnancies, it isn’t the only one. Selective Intrauterine Growth Restriction (sIUGR) occurs when there is an unequal placental share between the babies leading to abnormal blood flow. This causes slowed or even stopped growth for one baby.
If the growth of the smaller baby falls below the 10th percentile you will be monitored more often and depending on the severity delivery may be the best option. Learn more about sIUGR here.
Twin Reversed Arterial Perfusion sequence
Twin Reversed Arterial Perfusion sequence (TRAP) happens when the cardiac system of one twin fails to develop properly. The normally developing twin becomes a pump, doing double the work to pump for both babies. This puts extreme stress on the pump twin’s heart which can lead to cardiac failure. It is referred to as reversed arterial perfusion because the blood actually flows the wrong way.
Twin anemia-polycythemia sequence (TAPS) is a form of TTTS that can occur later in the pregnancy and is characterized by an imbalance in red blood cells and hemoglobin. The donor twin may be anemic and the recipient has too much blood and may require blood thinners after birth.
With TAPS you will likely see one twin born very pale and the other dark red. A Doppler ultrasound that measures the peak systolic velocity of the middle cerebral artery (MCA-PSV) can detect TAPS. Identical Twin Pregnancy
These complications are rare but even if you don’t develop them you are still not completely in the clear. Carrying twins means you are at higher risk for gestational diabetes, preeclampsia, and preterm labor.
Your specialist will monitor your cervix and fluid levels and if anything indicates preterm labor they can administer magnesium to stop it (before 32 weeks). They can also give steroid shots to help the babies’ lung development.
Sometimes though those babies are coming no matter what. Thankfully advances in medicine have come so far that twins born at just 24 weeks are surviving and thriving.
When to deliver?
If you do make it all the way without issues you will come to the question of when to deliver. If you have a great specialist this conversation will happen long before you even get close to delivery. Identical Twin Pregnancy
Nowadays, there is a huge push to go the full 40 weeks of pregnancy, and in normal situations this is absolutely the best, but this is not the case for MoDi pregnancies. The risk of complications and stillbirth increases substantially between weeks 33 and 38, and as a result the ACOG now recommends Mono Di twins deliver no later than 37 weeks & 6 days.
It is important to monitor even more frequently in the final few weeks (32+) to catch any problems and deliver as soon as possible. Proper monitoring in late pregnancy, including weekly scans and non-stress tests can ensure delivery before issues arise.
While early delivery can mean NICU time for your babies, it is far better than the alternative. My MFM required induction or scheduled c-section at 35 weeks, but the average delivery is between 36 and 37 weeks.
The reality is that after 8 months of supporting two babies the placenta can struggle to keep up, the babies can stop growing, and without intervention the worst can and does happen; though, thankfully, it is with much less frequency thanks to increased education among medical professionals on how to properly care for a Mono Di twins pregnancy. Identical Twin Pregnancy
You are also at increased risk of c-section or the dreaded “double whammy” in which baby A is delivered vaginally and baby B must be born via emergency c-section. Don’t worry too much about that though, plenty of women deliver both babies vaginally with no issues.
However, even in the most perfect conditions, complications can arise so be sure to find a good hospital with an excellent level-3 NICU and educate yourself on all scenarios beforehand.
If you’re like me you probably find yourself terrified with all that can go wrong and then on top of it it can be quite frustrating when your “birth plan” has to go out the window (I originally planned for an all natural home water birth assisted by midwives and a doula; I ended up with a scheduled c-section in an operating room).
But with proper care and consistent monitoring, the odds are that you will have an uncomplicated and wonderful experience! Identical Twin Pregnancy
The number one thing to remember is that being in the care of a skilled doctor who is well educated on the risks of Mono Di twins can make all the difference in your pregnancy and birth experience. Be your own advocate, fight for your babies, and do not settle for anything less than excellent care.
Take good care of yourself, drink lots of water, and eat a lot of healthy food. Move your body but rest as much as you can. Most of all, try your best to enjoy it, despite the anxiety. Most people only get to be pregnant with identical twins once in a lifetime and, while scary, it is a truly remarkable thing your body is doing!
Maigen is a former child care provider turned Blog-At-Home-Mom of 10-month-old identical twins, Juliette and Vivienne. She lives in upstate NY with her husband, the twinsters and their two cats, Boots and Willow. She enjoys reading, wine, being outside and spending time with her family and friends. She can be reached at via her blog Life and Love. Multiplied, on Facebook, Twitter and Instagram. For more articles on Twiniversity by Maigen, click here.