Monochorionic-monoamniotic twins: An overview to help you understand

I came across three really great journal articles containing very valuable information about monochorionic monoamniotic twins (which I will refer to as MoMo twins). I have read through these articles many times and decided to summarize them so that MoMo twins may be clearer to those who were unsure of exactly what is going on. Briefly, I would like to share our personal story thus far in this difficult journey, and then I will go on and provide some details from the journals about this particular type of twinning.

Tim and I found out that we were pregnant in early June of this year and needless to say we were very surprised. Little did we know at the time that there were many more revelations to come. Just as we were getting comfortable with the fact that we were going to be parents, we went in for our first ultrasound at nine weeks. We were excited to get the first glimpse at our tiny creation. The technician told us then that we were in fact having twins and that they looked to be in the same sac which usually meant they were monozygotic (identical) twins. You can probably imagine our reactions. Finding out we would be having two babies instead of one was exciting yet extremely overwhelming news to take in. Later the same day of our nine week ultrasound, the clinic called us to tell us that they were going to be contacting Rockford Memorial Hospital (RMH) for another opinion and to set up some ultrasounds with the Maternal Fetal Medicine Department. At twelve weeks we made the trip to RMH for another ultrasound. We learned for certain that day that we were pregnant with MoMo twins, which set us on a path that we are only now fully learning the implications of to this day. On September 25 we were delighted to find out that we will be having identical twin girls. We had a successful ultrasound with both heartbeats at 153 and 7 and 8 oz. So far so good!

To start out it is important to distinguish between monozygotic and dizygotic twinning. Monozygotic twins are derived from a single (mono) egg (zygote). Monozygotic twins form when a single fertilized egg splits into two embryos. Because the twins share the same DNA set, they tend to have similar features and are commonly known as identical twins. Dizygotic twins, on the other hand, are derived from two (di) eggs (zygotes). Dizygotic twins form when two separate eggs are fertilized by separate sperm and are more often known as fraternal twins. Every year 4 million infants are born in the United States and approximately 130,000 of them are twins. Only one-third of all twins are monozygotic, but 75% of the monozygotic twins are monochorionic. Among monochorionic twins, about 2% are monochorionic monoamniotic. It might help to define those main terms that I keep referring to. Monochorionic twinning happens when monozygotic twins are contained in a shared chorion (placenta). The chorion is the outer membrane of the sac surrounding a fetus in utero. Monochorionic twins can be either monoamniotic or diamniotic. Monoamniotic is a rare type of monozygotic twinning where the twins are enclosed within a shared amniotic sac (the inner membrane) in utero. Because there is no membrane separating the babies, they are at risk for cord entanglement. I will go into more detail about cord entanglement shortly, and I have attached images of what all four cases of twinning looks like to perhaps make this easier to see. (See attached images) MoMo pregnancies happen in about 1/25,000 to 1/60,000 pregnancies. The exact reason this happens in unknown, but it is known that it occurs as a result of late splitting of the developing embryo. The blastocyst is a thin-walled hollow structure in early embryonic development that contains a cluster of cells called the inner cell mass from which the embryo arises. Monoamniotic twinning is the result of splitting of the blastocyst between 8 and 13 days after fertilization, and occurs in approximately 1 to 2% of all monozygotic twin pregnancies. There has been a lot of research within the last twenty years that has changed outlook of the high fatality rate thought to be characteristic of MoMo twins. Mortality rates have been reported as high as 47%, but a recent review of the literature revealed a value of 10.8%.2 In much of the literature I have read the mortality rate of 10.8%-20% is a more accurate rate. This, however, is dependent on many factors which I want to go over in some detail. To begin it is important to understand some of the reasons that a MoMo pregnancy is at such high risk for fetal demise. Among these risk factors are umbilical cord entanglement and/or compression, twin-to-twin transfusion and preterm birth. It is known that uncomplicated twin pregnancies have a higher incidence of premature birth than singletons and that MoMo twins are at an even greater risk of being born before 32 weeks of gestation. The most serious and frequent complication that is seen in these specific pregnancies is fetal death due to umbilical cord entanglement. Cord entanglement, a condition unique to MoMo pregnancies occurs in 42-80% of the cases. As a result of the high probability of complications with MoMo pregnancies there is a lot of controversy in the type of management that should be applied during gestation. The most debated within the last twenty years, is whether there should be inpatient versus outpatient monitoring. This is, in fact, the basis of study in all three journal articles I have read. We have chosen to take the inpatient route for many reasons. After our consultation with the doctors and midwife at RMH, and my own research on the subject, I believe that being admitted into the hospital at 24 weeks is the best option to achieve the successful birth of our babies with the least amount of complications. Each of the studies showed a 100% success rate (success defined as babies who lived) in the patient groups that decided to be admitted as inpatients at 24-26 weeks gestation. The most recent and largest sample group was the study done by Heyborne, which is the one I would like to outline briefly. These professionals looked at 96 monoamniotic twin pregnancies from January 1993 to December 2003 that took place at eleven different universities and private perinatal practices. The primary outcome measure was intrauterine fetal death, in other words which babies died before delivery. They were moreover looking at whether the type of treatment could affect such outcome. They also measured gestational age at hospitalization (how many weeks pregnant the mother was), how many days the mother was in the hospital, gestational age at delivery, combined birth weight of the twins, combined neonatal intensive care days, combined days on the ventilator, and how many babies died. In this study 134 (of the 192 total babies) were female and 46 male infants. This is in agreement with the known predominance of female babies among monoamniotic twins. Of the 87 pregnancies (excluding those that were taken out of the study because of anomalies) there were 43 women admitted electively for inpatient fetal monitoring, and the remaining 44 were monitored as outpatients and admitted only for routine obstetric indications. I know that some of this may seem confusing, but the part that is most important to convey is the results of the study. No fetal deaths occurred in patients who were hospitalized electively for fetal monitoring. Among women followed as outpatients, there were 13 fetal deaths. The electively admitted group had a later gestational age at delivery, a high combined birth weight, and a decreased composite neonatal morbidity. Although one might expect that cord accidents occur abruptly and without warning, the data suggests otherwise. The improved outcome in the pregnancies with more aggressive fetal surveillance suggests that fetal deaths because of cord entanglement in monoamniotic twins are subacute events that can be predicted and prevented with monitoring 2 to 3 times a day. Although the optimal time for delivery is still controversial, most management strategies favor delivery no later than 32 to 34 weeks of gestation. The doctor informed us that they have seen in MoMo pregnancies that waiting too long past 32 weeks permits a more dangerous environment for the twins because of lack of space. Also, in each of the studies all the twins born successfully were delivered by c-section.
I tried to summarize my findings in such a way to make it easier to understand. I also felt strongly about trying to educate family and friends about monochorionic monoamniotic twins, because they are so uncommon. I will be admitted as a patient at RMH at 24 weeks, which is tentatively scheduled for November 6. Although this will be a very trying time for us, being away from each other, but we are willing to do anything in our power to bring these two tiny babies into the world kicking and screaming. As I am pretty much an expert in all things MoMo twins feel free to contact me if there are more questions.