Prenatal genetic testing requires access to fetal DNA. As described in commonly owned US Patent Appl. Publ. No. 2010/0159506, fetal genetic material can be found within fetal cells present in the mother's circulating blood. These fetal cells originate in the fetus and cross the placenta to enter the mother's circulatory system.
Maternal blood contains both nucleated (i.e., containing genetic material) and non-nucleated cells of both fetal and maternal origin. In order to focus attention on the cells of most interest, a first step in fetal genetic testing may therefore be to concentrate nucleated cells within the sample. One prior approach is described in US Patent Appl. Publ. No. 2010/0159506. Since red blood cells are denser than white blood cells, a preliminary separation of red blood cells is obtained by a single density gradient to separate mononuclear cells, including nucleated red blood cells, from a whole blood sample. The sample is then applied to a slide in a monolayer, stained, and analyzed.