I am not a doctor, just a mama of twins.  This is information I’ve learned from various sources during my pregnancy and should not be considered medical advice.  You should always talk to your health professional about any questions or concerns that you have.

Before getting pregnant with twins,  I would definitely have said that there are two types of twins: identical and fraternal.  Very few people know that this is not the end of the discussion when it comes to twin type or the implications.  When people say that twins are identical or fraternal they are referring to zygosity:  (1) identical twins are formed when a single egg is fertilized by a single sperm, and at some point, for unknown reasons, the egg splits and forms two babies; (2) fraternal twins, alternatively, are when two eggs are released and fertilized by two sperm.

So far, we’re following right along with what your high school biology book discussed, right?  When it comes to a twin pregnancy, however, whether they are identical or fraternal is of little consequence in your treatment and the babies’ prognosis.  What really matters is chorionicity and amnionicity.  In layman’s terms, what really matters is how many placentas and how many amniotic sacs are present. Here’s a helpful illustration that shows the differences:



1.  Dichorionic/Diamniotic (di/di) twins.  Di/di twins are the most common type of twins and the lowest risk.  Di/di twins can be identical if the egg split very early, but fraternal twins are always di/di.  In di/di twins, each twin has their own placenta and their own amniotic sac.  Di/di twin pregnancies have increased risks over singleton pregnancies, but this is the best case scenario in twin world.  The biggest worries tend to be going into preterm labor and making sure both babies are growing adequately.

2.  Monochorionic/Diamniotic (mo/di) twins.  This is what the Babies Blue are.  Mo/di twins share a placenta, but each baby has it’s own amniotic sac.  My doctors explained mo/di twin by saying they share a placenta and are in the same outer sac, but have a thin membrane dividing them which creates separate amnitoic sacs.  Mo/di twins are always identical, though there are some references to fused placentas, which can cause some confusion.  Mo/di twins are considered mid-risk in the twin world.  Anytime babies share a placenta, they are at risk for twin-to-twin transfusion syndrome (TTTS), which basically means the blood and nutrients from one baby (the donor twin) are going through the placenta to the other twin (the recipient twin).  In the past, developing TTTS resulted in devastating prognoses, but in recent years, thanks to development of new procedures, twins with TTTS have a much better chance of survival if it is caught early.  My maternal-fetal medicine specialist told us that there is a 1 in 7 chance of mo/di twins developing TTTS.

3.  Monochorionic/Monoamniotic (mo/mo) twins.  This is the least common type of twins and the highest-risk type.  Mo/mo twins are always identical.  They have one placenta and one amniotic sac.  They have the same risk for TTTS that mo/di twins have, but they also have risks associated with being in the same amniotic sac.  Particularly, as the babies move around one sac, their umbilical cords can become entangled or compressed, which can result in one or both babies not receiving enough nutrients and blood.

Knowing the type of twins you are having is critical because your treatment can and should greatly vary based on this information.  Di/di twins are followed more closely than a singleton pregnancy, particularly as you get closer to the end, but in many respects, your ultrasound and appointment schedule may look similar to that of a singleton pregnancy for the first few months.  If you are having mo/di or mo/mo twins, you should be followed much more closely to be monitored for TTTS and cord issues and you should be seen by a maternal-fetal medicine specialist in addition to your OB.  For TTTS, you should be seen for an ultrasound every other week from 16 weeks until the end of your pregnancy.  If doctors note any signs of TTTS, they will begin monitoring you more often or form a plan of treatment then.  For mo/mo, it is my understanding that you will be followed even more closely to watch for cord entanglement or compression.  This is generally a slow process, which gives parents and doctors time to make decisions.  Unfortunately, the only solution to these problems is delivery, so you and your doctors would have to weigh the risk of continuing the pregnancy versus the risk of premature birth.

The type of twins will also affect what type of health professional you can see.  For instance, some midwives will still take twin mamas, but generally only if they are di/di, which is at a much lower risk.

A twin pregnancy of any type can be an emotional roller-coaster for the parents.  Not only do you have an increased set of worries about how you will manage, how you will pay for daycare, if you will ever sleep again, but you have two of your babies’ safety and health to worry about also.  Just like any pregnant mom, you worry about whether your baby is doing okay, whether they have any problems, whether they will be born safely, only you are worrying about each individually, so arguably, double the worry.  Add in the fear about the increased risk of pre-term labor, and most twin moms seems to have a decent amount of anxiety.  If you’re having mo/mo or mo/di twins, the early months are possibly even more stressful than the later months because you are consistently being monitored for additional risks, and every ultrasound is just as terrifying as it is exciting.  Worrying is exhausting and when you start worrying so early in your pregnancy and never get a break (because the risk is always present),  it can definitely take a physical and emotional toll.  The later I get in my pregnancy, the more relaxed I am about TTTS because I know they have a much better chance of surviving outside the womb if they developed TTTS, but I find myself worrying more about preterm labor and researching each week how they would likely do if they were born at that point.  It is really very helpful to find a group of other moms of multiples who you can talk to through the ups and downs.  Having twins is an incredible gift, but sometimes we need some help to focus on that instead of all the risks.

Generally, no.  I think whether you are likely to end up on bed rest is largely based on (1) your doctors’ philosophy on bed rest; and (2) whether you develop TTTS, preeclampsia, go into preterm labor, etc.  My doctors are fairly anti-bed rest, so I don’t anticipate that at this point.  I have heard that mo/mo twin mamas are more likely to be placed on bed rest just because of the type of twins, but I don’t have enough knowledge to address that here.

Yes.  Many di/di twin mamas will be allowed to go just as long as they possibly can.  Medical literature suggests that mo/di pregnancies not be allowed to go past 36 or 37 weeks.  This is because the placenta may begin to deteriorate, which can happen quite rapidly, resulting in the loss of one or both babies.  My doctors have told me multiple times that it is less risky to deliver our babies at 36 weeks and have them possibly need NICU time than it is for them to stay in me past 37 weeks at the latest.  It’s counter-intuitive that the uterus is not the safest place for your baby until they are 40 weeks, so this can be a hard pill to swallow when you’re first told that you won’t be permitted to go full-term.  Your doctor may insist that your mo/mo twins be delivered even earlier if they are having any cord issues.  All twin pregnancies are at an increased risk for pre-term labor, so regardless of the above information, you should know and watch for signs of labor and call your doctor if you have any concerns.  It’s better to be the person that calls 100 times than to be the person who waits too long and labor can’t be stopped.

Did you know there were so many facets to twin-type?  If you’re a twin parent, would you add anything to the major differences in types of twins?