The fetal fraction depends on maternal weight and gestational age and can reach very low thresholds in about 3% of pregnant women with a certain maternal weight (>80 kg) or a corresponding body mass index (Ultrasound Obstet Gynecol 2013 41:26-32). For this reason, this weight category may be incompatible with an examination performed in the very early stages of pregnancy (≤10 weeks' gestation). The fetal fraction obtained may prove to be too low to produce a result.

Twin pregnancies also represent special cases. The Prendia test allows an analysis in the context of a twin pregnancy. However, the sex of the fetuses cannot be determined, and in the case of an abnormality, the test cannot define which fetus(es) may be affected. An invasive prenatal diagnostic test is required for this.

Vanishing twin: When an empty embryo sac is present in addition to a progressing pregnancy, the context is the same as for a twin pregnancy. The fetal sex cannot be provided and the cause of any abnormality cannot be assigned. An invasive prenatal diagnostic test is required.

Feto-placental discrepancies: certain abnormalities may be confined to the placenta or, conversely, to the fetus. This non-invasive screening test alone cannot detect these possible genotype differences between the fetus and placenta. These biological factors could be responsible for false-negative and false-positive results. If there is any doubt, invasive prenatal diagnostic tests are required.

Other extremely rare causes of false-positive results include acute tumour disease in the mother and mosaic aneuploidy in the mother.

This test can also be performed in a pregnancy resulting from an egg donation or if the couple is inbred.

Constraints of prenatal diagnostics