Non-Invasive Prenatal Testing (NIPT) analyzes small fragments of cell-free DNA (cfDNA) from a pregnant person’s blood. This screening test identifies an increased chance of certain fetal chromosomal conditions, such as trisomy 21 (Down syndrome), trisomy 18, and trisomy 13. Performed after 10 weeks of gestation, NIPT provides a risk assessment, not a definitive diagnosis. High-risk results require further diagnostic testing for confirmation.
How NIPT Identifies Twin Pregnancies
NIPT analyzes cell-free DNA fragments, a mix of the pregnant person’s DNA and DNA from the placenta, which mirrors the fetus’s genetic profile. The proportion of placental cfDNA in the maternal bloodstream is called fetal fraction. In a twin pregnancy, the total fetal fraction is higher than in a singleton pregnancy, as it originates from two placentas.
While NIPT primarily screens for chromosomal conditions, an elevated fetal fraction or detection of genetic markers consistent with two distinct fetal DNA profiles can suggest a multiple gestation. For example, if the test detects both Y chromosome material (indicating a male fetus) and X chromosome material suggesting a second female fetus, it points toward a twin pregnancy. NIPT is not designed as the initial method to detect twins; its capability in this regard is a byproduct of its primary analysis.
Understanding NIPT Results in Twins
NIPT can screen for common aneuploidies like trisomy 21, 18, and 13 in twin pregnancies. The test analyzes combined fetal DNA from both twins. If an abnormality is detected, the result indicates an increased risk for the pregnancy as a whole, not for a specific twin.
Sex determination for each fetus is possible if twins are discordant (one male, one female). The presence of Y chromosome sequences indicates at least one male fetus; if the profile suggests two fetuses, the absence of a second Y chromosome implies the second is female. If both fetuses are the same sex, NIPT cannot differentiate their individual sexes. NIPT’s accuracy for aneuploidy detection is reduced in twin pregnancies compared to singleton pregnancies.
Limitations and Nuances for Twin Pregnancies
Applying NIPT to twin pregnancies introduces several complexities that can affect the accuracy of the results. One challenge is “vanishing twin syndrome,” where one twin is lost early in pregnancy. Residual cfDNA from the vanished twin can remain in the maternal bloodstream, potentially skewing NIPT results and leading to false positives for aneuploidies or incorrect sex determination for the surviving twin.
NIPT also cannot distinguish between monochorionic (sharing a placenta) and dichorionic (separate placentas) twins. This distinction is clinically relevant because monochorionic twins share a single placenta and are genetically identical, so an aneuploidy affects both. Dichorionic twins have separate genetic makeups, meaning one twin could be affected while the other is not, which NIPT struggles to differentiate.
Complementary Diagnostic Approaches
Ultrasound remains the definitive method for confirming the presence of a twin pregnancy, determining chorionicity (whether twins share a placenta), and assessing the health and development of each fetus. An early ultrasound, typically performed in the first trimester, is often recommended before NIPT to confirm viability and gestational age, and to identify multiple gestations. This initial imaging provides foundational information that NIPT cannot.
If NIPT results indicate a high risk for a chromosomal abnormality in a twin pregnancy, further diagnostic procedures may be recommended to confirm the findings. These procedures, such as amniocentesis or chorionic villus sampling (CVS), involve taking a sample of amniotic fluid or placental tissue directly from each fetus or placenta. These diagnostic tests provide a definitive chromosomal analysis for each twin, offering precise information that NIPT, as a screening tool, cannot.