The math surrounding how rare MoMo twins are not entirely straightforward, partially because some institutions calculate incidence rates based on total pregnancies (in the United States, most commonly) and others calculate them based on twin pregnancies. Let’s review some credible sources to figure out quite how rare they are.
In this 2006 Nature article, it mentions 130,000 twins are born in the United States every year, and that a third of them are monozygotic (thus identical), and 3/4 of the monozygotic twins are monochorionic (share the same placenta), of which 2% are also monoamniotic (share the same amniotic sac). That means 130,000*1/3*3/4*0.02 or about 650 MoMo twins are born every year in the United States.
Assuming roughly 4 million births that year, that would be less than 0.02% of all pregnancies in the US are MoMo, or about 0.5% of all twin pregnancies in the US, or about 1.5% of monozygotic twin pregnancies.
These papers supports the data above:
- Inpatient vs outpatient management and timing of delivery of uncomplicated monochorionic monoamniotic twin pregnancy: the MONOMONO study (paper)
- Management of monochorionic monoamniotic twins: case reports and a review of literatures (paper)
- Early Ultrasound Identification of Cord Entanglement in Monochorionic Monoamniotic Twin Pregnancy (paper)
Now it’s worth highlighting a few things that aren’t clarified in these numbers. For one, we’re assuming that the statistics are referring to a twin birth as 1 birth, not 2. We assume that MoDi twins that turn into MoMo are also counted as MoMo (at birth), and that fetal demise of one twin does not count.