Using a highly sensitive allele‐specific PCR amplification method, we have previously shown that maternal cells could be detected in all 10 cord bloods tested. This raised the question of whether maternal cells are released into cord blood during the process of delivery or whether they are already present during pregnancy. We have now used the same PCR method to detect the presence of maternal cells in nine fetal blood samples collected at different gestational ages. Maternal cells were detected in eight samples obtained between 24 and 35 weeks of gestation. They were estimated to amount between 10⁻⁴ and 10⁻⁵ of nucleated fetal blood cells. In two cases mononuclear and polymorphonuclear cell fractions were separated by Ficoll gradient centrifugation and maternal cells were detected as comparable levels in both fractions. Maternal cells could not be detected in the one fetal blood sample obtained at 20 weeks of gestation, suggesting that maternal cells could appear at detectable levels in fetal blood during the third trimester of pregnancy. These results are discussed in terms of materno‐fetal immune tolerance and of transmission of viruses (and more specifically of the human immunodeficiency virus) from mother to child.