The discovery of a twin pregnancy brings immediate excitement, followed by questions about the babies’ development and health. The first-trimester ultrasound is a moment of intense focus, where the sonographer looks for specific signs that determine the nature of the twin gestation. This early information is important because how the twins share the placenta and surrounding membranes directly influences the entire course of the pregnancy.
Early Indicators of Twin Structure
The number of yolk sacs visible on an early ultrasound is one of the initial clues used to predict the twins’ structural classification. A yolk sac is a small, fluid-filled structure that provides nourishment to the developing embryo before the placenta fully takes over. Because each embryo initially develops its own yolk sac, seeing two separate yolk sacs is generally a strong indicator that the babies are diamniotic.
The definitive classification relies on the chorion, which forms the outer placental membrane, and the amnion, which forms the inner sac surrounding the baby. The number of yolk sacs often corresponds to the number of amniotic sacs, but this is not always reliable, especially very early in the first trimester. For example, two yolk sacs may be visible in monochorionic twins before the dividing amniotic membrane is seen. The true classification rests on the number of chorions (placentas) and amnions (inner sacs), not solely on the yolk sacs.
Understanding Twin Classification by Sacs
Twin pregnancy classification is based on chorionicity (the number of placentas) and amnionicity (the number of inner amniotic sacs). This classification system is used to understand the potential risks and determines the specialized monitoring required throughout the pregnancy. All twin pregnancies fall into one of three structural categories, depending on whether two separate eggs were fertilized or how early a single fertilized egg splits.
The least complex type is Dichorionic-Diamniotic (Di-Di), which accounts for the majority of all twin pregnancies and is the structure for all fraternal (dizygotic) twins. Each baby has its own separate placenta (dichorionic) and its own separate inner sac (diamniotic). Identical (monozygotic) twins can also be Di-Di if the single fertilized egg splits very early, within the first three days after conception.
A common structure for identical twins is Monochorionic-Diamniotic (Mo-Di), occurring when the single fertilized egg splits between four and eight days after fertilization. These twins share a single placenta (monochorionic) but are separated by individual inner amniotic sacs (diamniotic). Since they share a placenta, Mo-Di twins are always identical and account for about 70% of all identical twin pregnancies.
The highest-risk classification is Monochorionic-Monoamniotic (Mo-Mo), which is always identical and happens when the egg splits after nine days. These twins share a single placenta (monochorionic) and a single inner amniotic sac (monoamniotic), meaning there is no dividing membrane between them. Mo-Mo twins are rare, accounting for about 1% of identical twins, but the lack of a separating membrane presents unique risks.
Management Differences Based on Classification
Knowing the structural classification dictates the entire schedule and intensity of prenatal care. The sharing of a placenta (monochorionicity) is the main factor that increases the risk profile. Dichorionic-Diamniotic (Di-Di) twins have the lowest risk profile, and monitoring is typically less frequent, with growth scans often recommended every four weeks starting around 20 weeks.
Monochorionic pregnancies (Mo-Di and Mo-Mo) require closer surveillance due to the risk of conditions like Twin-to-Twin Transfusion Syndrome (TTTS). TTTS involves unequal blood flow in the shared placenta, which can lead to one twin receiving too much blood and the other too little. Monochorionic-Diamniotic (Mo-Di) twins receive ultrasound scans every two weeks starting from 16 weeks gestation to screen for TTTS and other complications.
Monochorionic-Monoamniotic (Mo-Mo) twins face the highest risk because, in addition to TTTS, the absence of a dividing membrane allows the umbilical cords to become entangled. Cord entanglement can restrict blood flow and is a serious concern. This often leads to recommendations for inpatient admission with daily monitoring starting between 24 and 28 weeks of gestation.