Introduction: We have established a robust method to specifically identify and isolate a placental fetal cell in maternal blood (fcmbs) at a gestational age of 12 weeks. The concentration of these cells, however, varies considerably among pregnant women (median 3 fcmbs/30 mL blood, range 0-18), which is a challenge for the implementation of the method for prenatal diagnostic purposes. We conducted a study to identify factors influencing the number of fcmbs at a gestational age of 11-14 weeks. Methods: 59 pregnant women at weeks 11-14 were included, and information about lifestyle and daily activity was obtained by a questionnaire and a structured interview. The number of fcmbs was assessed in 30 mL blood processed by a proprietary method developed in-house. Fetal cells in the blood, binding to fetal cell specific antibodies, were initially isolated by magnetic cell sorting. The fetal cells were then stained with a cocktail of fetal cell-specific antibodies, identified and counted. Results: Participants carrying male fetuses had higher median number of fcmbs per 30 mL blood than those carrying female fetuses (5 vs. 3, p=0.004). Exercise within 3 hours (1.5 vs. 4, p=0.02) and 24 hours (2 vs. 4, p=0.06) decreased the number of fcmbs, whereas coitus the evening before increased the number (4 vs. 3, p=0.11). Conclusion: The number of fcmbs is affected by normal activities. This should be taken into account when planning collection of fetal cells in connection for prenatal diagnosis. The association of fetal gender might reflect yet unknown aspects of placentation.