Results of noninvasive prenatal RHD testing in Gestation Week 25 are not affected by maternal body mass index

BACKGROUND Reliability of noninvasive prenatal RHD genotype test (NIP RHD) depends on having sufficient amounts of cell‐free fetal DNA (cffDNA) in the maternal plasma sample. The fraction of cffDNA in maternal plasma is inversely related to maternal body mass index (BMI), suggesting that high maternal BMI may limit the test's accuracy. This study determined the effect of maternal BMI on the accuracy of NIP RHD. STUDY DESIGN AND METHODS Results from NIP RHD performed in Gestation Week 25 were correlated to maternal BMI in Week 12. The accuracy of NIP RHD result was determined by correlation with serologic RhD types of the neonates. RESULTS A total of 1618 pregnancies in 1588 D– women were included. Median BMI in these pregnancies was 24.2 (10%‐90%, 20.1‐32.4), and in 261 of 1618 (16%) pregnancies BMI was 30 or more (median BMI in this group was 33.6; 10th‐90th percentiles, 30.5‐41.1). NIP RHD was positive in 987 of 1618 (61%), negative in 582 of 1618 (36%), and inconclusive in 49 of 1618 (3.0%). Compared to the neonate's serologic RhD type, nine of 987 (0.9%) positive NIP RHD results were false positive, and four of 582 (0.7%) negative NIP RHD results were false negative (FN). In five of 49 (10%) inconclusive NIP RHD results, the neonatal RhD type was positive. There was no difference in median BMI between individuals who tested inconclusive or FN compared to those with true positive or true negative results (p = 0.80). CONCLUSION The accuracy of NIP RHD testing performed in Gestation Week 25 does not depend on maternal BMI in the 12th gestation week.