ARIGA ET AL. 1524 TRANSFUSION Volume 41, December 2001 www.transfusion.org BACKGROUND: Fetal genetic material is detectable in the maternal circulation and has been used for noninvasive prenatal diagnosis. However, few data are available con- cerning its quantity and natural history during gestation. STUDY DESIGN AND METHODS: This study prospec- tively characterized the kinetics of cellular and cell-free fe- tal DNA in the circulation of 25 healthy women during and after uncomplicated pregnancy. Real-time kinetic PCR was used to quantitate human Y-chromosome sequences, and liquid oligomer hybridization with 32 P-labeled probes was used to verify the identity of amplified products. RESULTS: In all male pregnancies, but no female preg- nancies, low-level fetal Y-chromosome DNA was detected in both cellular and cell-free compartments beginning at 7 to 16 weeks but increasing steadily after 24 weeks and reaching a peak at parturition. The fetal DNA decreased rapidly after birth. CONCLUSION: Fetal genetic material can be detected throughout pregnancy, and its quantity is a function of ges- tational age and of whether the plasma or cellular compart- ment is examined. Both the absolute quantity of fetal DNA and its ratio to total DNA (maternal + fetal) are greater in the plasma than in the cellular compartment. Fetal DNA is cleared rapidly from both compartments after parturition, which suggests that turnover is dynamic. Because they provide prospective and quantitative data concerning fetal DNA levels, these observations and kinetic PCR methods may have implications for noninvasive prenatal diagnosis. Further studies will be needed to determine the immuno- logic implications of fetal-maternal DNA exchange and cel- lular microchimerism. A lthough many studies support the concept that a physical and immunologic barrier exists at the maternal-fetal interface, little is known about how the semi-allogeneic fetus is protected from at- tack by the maternal immune system. Bi-directional cell traf- ficking across the maternal-fetal interface may play a role in this process. 1-6 Maternal cell migration into the fetus can in- duce fetal tolerance to maternal antigens in animal models 7 and in humans. 8 In addition, fetal progenitor cells can persist in the maternal circulation for many years after delivery, 9,10 and such fetal microchimerism may be etiologically associ- ated with maternal autoimmune diseases such as systemic sclerosis 11,12 and rheumatoid arthritis. 13 To date, the types of fetal cells detected in maternal blood include trophoblast, nucleated RBCs, WBCs, and HPCs. 14-20 Fetal nucleated RBCs have been identified as early as 7 weeks of gestation, although the reported frequencies of fetal nucleated RBC(s) during progressive stages of gestation have varied. 21-27 Some investi- gators have shown that nucleated RBC(s) increased as preg- nancy progresses, 22,23 whereas others have found that the fre- quency of fetal nucleated RBC(s) decreased and even became undetectable after the second trimester. 24,25 Trophoblasts may Kinetics of fetal cellular and cell-free DNA in the maternal circulation during and after pregnancy: implications for noninvasive prenatal diagnosis Hiromichi Ariga, Hitoshi Ohto, Michael P. Busch, Shinya Imamura, Robert Watson, William Reed, and Tzong-Hae Lee ABBREVIATIONS: CFS = charge flow separation; C T = thresh- old cycle; kPCR = kinetic PCR; MACS = magnetic-activated cell sorting; SRY = sex-determining region of the Y chromosome. From the Blood Centers of the Pacific; and the Department of Laboratory Medicine, University of California, San Francisco, San Francisco, California; Fukushima Medical University School of Medicine; and Odaka Hospital, Fukushima, Japan; Blood Systems, Scottsdale, Arizona; and Roche Molecular Sys- tems, Alameda, California. Address reprint requests to: Tzong-Hae Lee, MD, PhD, Blood Centers of the Pacific, 270 Masonic Avenue, San Fran- cisco, CA 94118; e-mail: thlee@qpcr.com. Supported in part by a Specialized Center of Research (SCOR) Grant in Transfusion Medicine from the National Heart, Lung, and Blood Institute and by a grant from Roche Molecular Systems. Received for publication March 15, 2000; revision re- ceived July 14, 2001, and accepted July 25, 2001. TRANSFUSION 2001;41:1524-1530. T R A N S F U S I O N M E D I C I N E