We have developed a real-time quantitative PCR assay to measure the concentration of fetal DNA in maternal plasma and serum. Our results show that fetal DNA is present in high concentrations in maternal plasma, reaching a mean of 25.4 genome equivalents/ml (range 3.3-69. 4) in early pregnancy and 292.2 genome equivalents/ml (range 76. 9-769) in late pregnancy. These concentrations correspond to 3.4% (range 0.39%-11.9%) and 6.2% (range 2.33%-11.4%) of the total plasma DNA in early and late pregnancy, respectively. Sequential follow-up study of women who conceived by in vitro fertilization shows that fetal DNA can be detected in maternal serum as early as the 7th wk of gestation and that it then increases in concentration as pregnancy progresses. These data suggest that fetal DNA can be readily detected in maternal plasma and serum and may be a valuable source of material for noninvasive prenatal diagnosis.

Fetal DNA has been detected in maternal plasma during pregnancy. We investigated the clearance of circulating fetal DNA after delivery, using quantitative PCR analysis of the sex-determining region Y gene as a marker for male fetuses. We analyzed plasma samples from 12 women 1-42 d after delivery of male babies and found that circulating fetal DNA was undetectable by day 1 after delivery. To obtain a higher time-resolution picture of fetal DNA clearance, we performed serial sampling of eight women, which indicated that most women (seven) had undetectable levels of circulating fetal DNA by 2 h postpartum. The mean half-life for circulating fetal DNA was 16.3 min (range 4-30 min). Plasma nucleases were found to account for only part of the clearance of plasma fetal DNA. The rapid turnover of circulating DNA suggests that plasma DNA analysis may be less susceptible to false-positive results, which result from carryover from previous pregnancies, than is the detection of fetal cells in maternal blood; also, rapid turnover may be useful for the monitoring of feto-maternal events with rapid dynamics. These results also may have implications for the study of other types of nonhost DNA in plasma, such as circulating tumor-derived and graft-derived DNA in oncology and transplant patients, respectively.

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Noninvasive fetal RhD genotyping can be performed rapidly and reliably with the use of maternal plasma beginning in the second trimester of pregnancy.