Introduction: Fetal cells in maternal blood may be used for noninvasive prenatal diagnostics, although their low num- ber is a challenge. This study’s objectives were to evaluate whether physical activity, transabdominal and transvaginal ultrasound scans of the uterus, as well as overnight or day- to-day variation affect the number of isolated fetal cells, more specifically the presumed endovascular trophoblast (pEVT). Material and Methods: In each of 3 different experi- ments, 10 normal singleton pregnant women (gestational age 10+4–14+4 weeks) participated. The number of pEVTs was assessed in 30–36 ml blood using specific markers for enrichment and identification. Results: The number of pEVTs increased overnight (p = 0.001) from a median of 1.5 to 3.5 and even further to a median of 6.0 after 30 min of phys- ical activity (p = 0.04) but was not affected by transabdomi- nal and transvaginal ultrasound scans. Repeated sampling showed that the interindividual variation of pEVTs was high- er than the intraindividual variation (p < 0.001). However, even in pregnant women with a consistently low number of
pEVTs, isolation of the pEVTs for prenatal diagnoses was pos- sible in all cases by doing 2 separate blood samplings a few days apart. Discussion: The number of pEVTs identified in maternal blood can be increased by presampling conditions or repeated sampling.