Most prenatal tests analyse a mix of maternal and placental DNA, and cannot distinguish the source of any abnormality (D. W. Bianchi Nature 522, 29–30; 2015). Our test, which was used to screen 185,000 women worldwide last year, reports specifically on fetal chromosomal status and so gets around the problem of validating incidental discoveries related to the mother's health.

The test detects single nucleotide polymorphisms (SNPs) in maternal and fetal DNA. The results are used to calculate a pregnancy-specific risk score; the maternal contribution is used only as input to the fetal-risk calculation (see B. Zimmermann et al. Prenat. Diagn. 32, 1233–1241; 2012). Therefore, incidental findings of maternal risk for cancer or abnormal numbers of sex chromosomes are not an issue. An exception is the partial deletion of chromosome 22 associated with DiGeorge syndrome. By providing the fetal-risk score, the test may identify the mother as a carrier of this deletion, as our consent form mentions.

Without knowing the sensitivities, specificities and positive or negative predictive values of incidental maternal findings, their clinical significance and utility are unknown. For example, there is currently no way to estimate residual risk when tests are negative for maternal cancer.