26. Fetal Cells and Breast Cancer. A number of fetal stem cells may cross the placenta from the fetus to the mother during pregnancy and remain in the mother's tissue for decades. A recent study shows that the presence of fetal cells in the mother may offer some protection against the onset of breast cancer.¹⁶ Of the 54 women in the study with breast cancer, 14 had fetal cells. Of the 45 women without breast cancer, 25 had fetal cells. Test whether the population proportions of women with fetal cells is lower among women with breast cancer compared with women without breast cancer, using level of significance .