Table 1.

Types of fetal cells found in maternal blood.

	Trophoblasts	Leukocytes	NRBCs	HSCs	MSCs	PAPCs
Specific cell markers	No, although HLA-G has potential	Yes	Yes, improving with use of embryonic hemoglobins	Yes	Yes, although defined by absence of markers	CD34+CD38+Endothelial
Similarity to maternal cells	None	Yes	Yes, with exception of primitive fetal NRBCs	Yes, but have higher proliferative potential	None in blood	Not known
Found in non-pregnant adult circulation	No, cleared by pulmonary circulation	Yes	Yes	Not typically	No	No
Persist in maternal tissues	No	Yes	No, short-lived	Yes	Yes	Yes
Other qualities	Developmentally end-stage, Multinucleated	Identified by HLA differences between mother and fetus	Physiologic increase in maternal NRBCs occurs during pregnancy	Rare, but can be amplified in vitro	Rare, but can be amplified in vitro, may escape immune rejection. Likely to adhere and engraft rapidly	Have capability of differentiation to repair maternal tissues

Abbreviations: NRBCs, nucleated erythrocytes; HSCs, hematopoietic stem cells; MSCs, mesenchymal stem cells; PAPCs, pregnancy-associated progenitor cells; HLA, human leukocyte antigen.