The Non-Invasive Prenatal Test (NIPT) analyzes cell-free fetal DNA (cfDNA) circulating in the maternal bloodstream. This screening tool is widely used in singleton pregnancies to assess the risk of chromosomal conditions and determine fetal gender. Applying NIPT to a twin pregnancy is more complex because the test must account for the genetic material of two developing fetuses. Understanding NIPT performance for twin gender determination requires examining the biological mechanism, statistical confidence, and variables introduced by different twin types.
How NIPT Determines Fetal Gender
NIPT analyzes a sample of the mother’s blood, which contains fragments of her own DNA and cfDNA shed from the fetuses’ placentas. This cfDNA mirrors the fetuses’ genetic makeup. The fundamental mechanism for determining gender involves looking for the presence or absence of the Y chromosome.
A male fetus possesses an X and a Y chromosome, while a female fetus has two X chromosomes. If NIPT detects Y chromosome sequences in the mother’s blood, it indicates that at least one fetus is genetically male. Conversely, if the test fails to detect any Y chromosome material, the conclusion is that both fetuses are genetically female.
Statistical Accuracy for Twin Gender Determination
NIPT is highly accurate for gender determination in twin pregnancies, though interpretation depends on the result. When the test indicates both fetuses are female (no Y chromosome detected), the accuracy is exceptionally high. This high confidence level exists because the absence of the Y chromosome is a clear negative finding, suggesting a female-female twin pair.
Interpretation is more complex when the test detects the Y chromosome, indicating at least one male fetus is present. In this scenario, the test cannot distinguish between a male-male pair and a male-female pair, which is a limitation of standard NIPT. Accuracy for detecting the presence of a male is slightly lower than for two females, with sensitivities around 98% and specificities around 95%.
Why Twin Types Influence NIPT Results
The biological relationship between the twins, referred to as zygosity, introduces variables that influence NIPT results. The two main types are monochorionic and dichorionic twins.
Monochorionic twins, typically identical, develop from a single fertilized egg and share a placenta, meaning their genetic material is the same. Because their DNA is identical, NIPT analysis for gender is similar to that of a singleton pregnancy, maintaining high accuracy.
Dichorionic twins, which can be fraternal or identical, each have their own separate placenta, and their genetic makeup is often different. The cfDNA from each twin mixes in the mother’s bloodstream, and the individual contribution from each fetus can vary. If one twin sheds significantly less cfDNA than the other (unequal fetal fraction), it can potentially skew results, especially if the male fetus is shedding less DNA.
Practical Interpretation of Ambiguous NIPT Outcomes
When a Y chromosome is detected, the NIPT result confirms the presence of at least one male fetus, but does not provide the individual gender of each twin. Ambiguous results can also arise from a “vanishing twin,” where a demised fetus continues to shed its DNA for weeks. This can potentially cause a false male reading if the vanished twin was male.
If results are inconclusive or inconsistent with clinical findings, follow-up confirmation is necessary. Genetic counseling helps parents understand the complexities of NIPT results in multiple pregnancies. A detailed ultrasound is commonly used to confirm the gender of each twin later in the pregnancy.