Repeat cell-free DNA screening after initial false-positive results and term placental analysis for confined mosaicism: a prospective cohort study

False-positive prenatal cell-free DNA screening (cfDNA) results may arise from confined placental mosaicism (CPM). This cohort study examines the persistence of high-risk cfDNA in the third pregnancy trimester after exclusion of fetal involvement, and the concordance of these results with CPM in the postpartum placenta. Pregnant individuals receiving a false-positive primary cfDNA result were recruited from Monash Health and Monash Ultrasound for Women in Melbourne, Australia, between August 2023 and December 2024. Participants underwent genome-wide repeat cfDNA screening (r-cfDNA) after 30 + 0 weeks' gestation. Placental samples were collected and cytogenetically analysed postpartum. This cohort included 21 individuals, of which 33.3% (7/21) screened high-risk on r-cfDNA. There was no significant association between r-cfDNA and CPM in the postpartum placenta (p = 0.397), as five (5/12, 41.7%) cases involving CPM were screened 'low-risk'. Fetal-fraction was significantly lower (5.0% [IQR = 4.6-8.0%] vs. 9.0% [IQR = 7.0-13.5%], p = 0.025), and maternal BMI (Kg/m2) higher (29.0 [IQR = 25.7-31.2] vs. 23.3 [IQR = 22.0-24.9], p = 0.006), in false-positive cases not attributable to CPM detected by r-cfDNA or postpartum. High-risk r-cfDNA results were associated with smaller babies (median birthweight percentile = 12.3 [IQR = 4.4-21.7] vs. 31.9 [IQR = 21.5-55.5], p = 0.009), though the majority of outcomes were favorable. High-risk r-cfDNA is not a sensitive predictor of CPM, though high mosaic ratios are associated with adverse obstetric outcomes. An alternative explanation for false-positive cfDNA besides CPM may be low fetal fraction, particularly in the context of high maternal BMI.