During a twin pregnancy, you may have heard the terms “Di/Di” or “Mo/Di” used to describe twins. These terms are used to classify and explain twin anatomy in the womb. Although it may sound like a lot of medical gobbledy-gook, they’re important concepts for parents because some twins are at risk for complications.
Twins in the Womb
Let’s start by understanding the environment. In the womb, a fetus grows within its mother’s uterus. The placenta attaches to the wall of the uterus and serves the dual purpose of removing waste and providing nutrients to the fetus. It connects via the umbilical cord to the baby. The fetus is enclosed in a fluid-filled sac surrounded by a thin membrane, called the amnion. A second membrane, called the chorion, surrounds the amniotic sac. Let’s break it down:
Mo: Means one Di: Means two Chorion: Is the outer membrane Amnion: Is the inner membrane, which forms a sac filled with amniotic fluid containing the fetus
Mo Twin Terms
These terms describe twins—always monozygotic twins—that share an environment. They are enclosed in ONE amniotic sac and/or ONE chorion:
Monochorionic: One chorionMonoamniotic (or monoamnionic): One amnion
Di Twin Terms
These terms describe twins—which can be monozygotic or dizygotic—that each have a separate sac. There are two placentas, although in some cases, the placentas fuse together into one:
Dichorionic: Two separate chorionsDiamniotic: Two amnion/two amniotic sacs
Combinations
So let’s look at them in combination. The majority of twins are di/di, that is, dichorionic, diamniotic. That means that they developed separately, each with their own placenta and own sacs. This is how all dizygotic, or fraternal twins develop. In addition, some monozygotic (identical) twins will also form this way, depending on how early after conception the egg splits. However, some monozygotic twins will be mo/di or mo/mo. That means they are monochorionic but diamniotic, or they are monochorionic and monoamniotic. Twins cannot be di/mo. If there are two chorions, then they can not be in the same amniotic sac since it is the innermost membrane and, by definition, they will be separated. Let’s break it down further.
Di/Di (dichorionic/diamniotic): These are twins that are separate, with separate chorions and in their own amniotic sac. (Sometimes designated as DCDA.)Mo/Di (monochorionic/diamniotic): These are twins that are in separate amniotic sacs but contained within the same outer membrane. There is a single, shared placenta. (Sometimes designated as MCDA.)Mo/Mo (monochorionic/monoamniotic): These are twins that are in a single amniotic sac, with a single chorion. There is a single, shared placenta. (Sometimes designated as MCMA.)
How Is Twin Chorionicity Identified?
During pregnancy, an examination of the placenta(s) and membranes via ultrasound can sometimes confirm chorionicity. An ultrasound scan, usually early in the pregnancy, will check for one or two placentas and the thickness of membranes. A thicker membrane can indicate di/di twins, while a thin membrane or the absence of a membrane can signal mo/di or mo/mo twins. Sonographers can also check for the “twin peak sign," also called the “lambda sign” because of its resemblance to the Greek letter λ. It forms where the two placentas and sacs abut in di/di twins. A T-shaped structure can indicate mo/di twins. The only definitive confirmation of di/di twins during pregnancy, however, is when the babies are identified as one boy and one girl. If they are different sexes, they are dizygotic, with rare exceptions. Remember, all dizygotic twins are di/di, while monozygotic twins can be di/di, mo/di or mo/mo.
What Are the Ramifications?
Knowing the chorionicity of twins can help assess risk and determine a treatment plan to provide proper prenatal care for an optimal outcome of the twin pregnancy. Dichorionic/diamniotic (di/di) twins generally present fewer additional risks. However, mo/di and mo/mo twins should be monitored carefully because they are at increased risk for complications. Twin-to-twin transfusion syndrome (TTTS) or twin anemia-polycythemia sequence (TAPS) can develop in mo/di twins. Mo/mo twins can be compromised by cord entanglement.
