A twin pregnancy raises the question of whether the babies are identical or fraternal, a distinction important for both parents and medical care. The difference between twins is based on their genetic origin, determined by a process occurring early in development. While prenatal imaging, specifically ultrasound, cannot offer direct genetic analysis, it provides highly informative visual evidence. The features seen on an ultrasound allow healthcare providers to make a strong determination about the twins’ genetic relationship and plan care accordingly, even though absolute genetic proof often requires testing after birth.
Understanding Zygosity and Chorionicity
The terms used to classify twins refer to two separate biological concepts: zygosity and chorionicity. Zygosity describes the genetic makeup, determining if twins are identical or fraternal. Monozygotic twins develop from a single fertilized egg (zygote) that splits, sharing virtually the same DNA and are considered identical. Dizygotic twins, or fraternal twins, result from two separate eggs fertilized by two separate sperm, making them genetically no more alike than other siblings.
Chorionicity describes the structure of the placenta and the outer membranes, which is the physical arrangement visible during pregnancy. Dizygotic twins always have their own placenta and separate outer sacs, known as dichorionic-diamniotic (DCDA) placentation. Monozygotic twins can have one of three different placental arrangements depending on when the single fertilized egg divides. While zygosity determines if the twins are identical, chorionicity is the physical factor observed prenatally to infer zygosity.
Visual Clues on Ultrasound
The primary method for inferring twin zygosity is by determining chorionicity using ultrasound, ideally performed before 14 weeks of gestation. Dichorionic-diamniotic (DCDA) twins, whether fraternal or a type of identical, have two separate placentas or a fused placenta with a thick dividing membrane. The presence of two different fetal sexes is a definitive sign of dizygotic (fraternal) twins.
In same-sex DCDA twins, the “Lambda sign” or “Twin Peak sign” is often visible where the dividing membrane meets the placenta. This sign appears as a triangular wedge of placental tissue extending into the base of the inter-twin membrane, confirming a dichorionic pregnancy. Approximately one-third of identical twins are DCDA, but all fraternal twins are DCDA. This finding suggests the twins are either fraternal or a less common type of identical.
The presence of a shared placenta, known as monochorionic (MC) placentation, provides the strongest indication of identical twins. Monochorionic-diamniotic (MCDA) twins share a placenta but have separate inner sacs, while monochorionic-monoamniotic (MCMA) twins share both the placenta and the inner sac. In monochorionic pregnancies, the junction of the thin dividing membrane with the placenta forms a distinct “T-sign.” This T-sign occurs where the membrane meets the placenta at a perpendicular right angle without any extension of placental tissue. Since monochorionic twins develop from a single fertilized egg, this finding indicates the twins are identical.
Accuracy and Definitive Postnatal Testing
While prenatal ultrasound is highly accurate for determining chorionicity, it is not definitive for determining zygosity. Monochorionic twin pregnancies are considered identical, as the shared placenta arises from the splitting of a single zygote. The challenge arises with dichorionic-diamniotic (DCDA) twins, which account for the majority of all twin pregnancies.
DCDA twins are either dizygotic (fraternal) or represent about 20 to 30 percent of monozygotic (identical) twins who split early enough to develop separate placentas. If the twins are different sexes, they are definitively fraternal. If they are the same sex, however, ultrasound cannot distinguish between fraternal and identical twins. In these cases, the final determination of zygosity is usually made after birth.
The most reliable method for postnatal confirmation is DNA testing, typically involving a simple cheek swab from each twin. This test compares specific genetic markers to determine if the twins are a 100 percent match, confirming they are identical. Parents may observe physical similarities or differences, but DNA testing offers an objective, precise conclusion when the information is desired.
Medical Management Based on Early Identification
Identifying chorionicity early in pregnancy is a priority for medical professionals because it dictates the level of monitoring and specialized care required. Monochorionic pregnancies, where the twins share a placenta, present a higher risk for certain complications compared to dichorionic pregnancies. This shared placental circulation can lead to conditions requiring more frequent and intensive surveillance.
A major concern in monochorionic twins is Twin-to-Twin Transfusion Syndrome (TTTS), where blood passes unequally between the fetuses through connecting vessels in the shared placenta. Other risks include selective fetal growth restriction and twin anemia polycythemia sequence. To manage these risks, monochorionic pregnancies typically require ultrasound assessments every two to three weeks, beginning as early as 16 weeks of gestation.
Dichorionic twin pregnancies, where each twin has its own separate placenta, generally have fewer unique complications. They are followed more closely than singleton pregnancies due to the overall risks associated with carrying multiples, but the monitoring schedule is less intense than for monochorionic twins. Early identification of chorionicity allows healthcare teams to tailor the surveillance schedule and intervene promptly if high-risk conditions, such as TTTS, begin to develop.