Any discussion on commercial cell-free DNA (cfDNA) NIPT testing must consider the following points:
- Fetal fraction: Only a small proportion of the total cfDNA in the maternal blood is derived from the fetus (about 10%) and more precisely derived from the placenta. Thus, the accuracy of any cfDNA NIPT assay is dependent upon the fetal fraction of the total cfDNA existing in maternal blood. Most laboratories typically set a limit of approximately 4%.
- Gestation age: Although cffDNA can be found in maternal blood as early as four weeks gestation, the fetal fraction may not be present in adequate amounts for NIPT testing to be valid. Thus, if NIPT testing is done prior to 10 weeks, test results may be inaccurate because of the inability to detect cffDNA in the maternal blood. This can possibly produce an unacceptable result or even a false negative result. Except for the Panorama® NIPT assay, most NIPT assays recommend testing on pregnant women of 10 weeks or more gestation to ensure adequate levels of cffDNA.
- Maternal size: Even with NIPT testing occurring after 10 weeks gestation, the fetal fraction may still be too low to produce an acceptable result. Maternal size, especially a high maternal body weight, can cause a dilution effect resulting in a lower fetal fraction producing a possible failed result or even a false negative result.
- Confined placental mosaicism (CPM) and vanishing twin syndrome: As previously discussed, CPM represents a discrepancy between the chromosomal makeup of the cells in the placenta and the cells in the fetus. The presence of CPM can cause a false positive result to occur when performing NIPT testing. Vanishing twin syndrome occurs when a fetus in a multiple pregnancy dies in utero and is then partially or completely reabsorbed. The presence of a vanishing twin can also cause a false-positive result.
- Twin or multiple gestations: Twins occur in about 3% of all live births. Typically, cfDNA screening in twin pregnancies presents unique challenges. Although the total fetal fraction in twins is approximately 1.6 times that reported in singletons, the average fetal fraction per twin tends to be decreased. Most commercially available NIPT tests now offer increased analytical sensitivity and claim to be able to provide NIPT testing for twins; however, several professional organizations and major insurers suggest that there is still limited evidence about the performance of NIPT as a test for fetal aneuploidy in twin or triplet pregnancies. As noted in a later section, the recent ACOG guidelines do indicate that cfDNA screening can be performed in twin pregnancies.
