A prospective analysis of cell-free fetal DNA concentration in maternal plasma as an indicator for adverse pregnancy outcome.

OBJECTIVES

To evaluate whether cell-free fetal (cff) DNA in maternal plasma during the second trimester is a marker for developing pregnancy-associated complications. Two PCR techniques for the detection and quantitation of fetal DNA were compared.

METHODS

Plasma samples were prospectively collected from 84 pregnant women carrying male fetuses before amniocentesis (14-29 weeks). We later recorded 26 pregnancies with complicated outcomes, including five cases of fetal chromosomal abnormalities. For statistical analysis, two overlapping subgroups A and B were made. Each group was separately compared for total and fetal DNA with a corresponding group considered normal using Wilcoxon rank sum test. Male fetal DNA concentration in maternal plasma was quantified using real-time quantitative polymerase chain reaction (PCR) of SRY sequences. The samples were also analyzed by quantitative fluorescent PCR (QF-PCR) using highly polymorphic short tandem repeat DNA sequences (STRs), and the percentage of relative fetal allele concentration in maternal alleles was calculated and compared to the fetal/total DNA ratio obtained by real-time PCR.

RESULTS

Quantities of total and fetal circulating DNA were significantly correlated (r(2) = 0.44, P < 0.0001) with a median total DNA concentration of 522 GE/mL (range 51-3047) and a median fetal DNA concentration of 8 GE/mL (range 0-879). Neither level was correlated with gestational age in pregnancies with normal (r(2) = -0.05; P = 0.66, and r(2) = 0.02; P = 0.88, respectively) and abnormal (r(2) = 0.45; P = 0.17, and r(2) = 0.11; P = 0.76, respectively) outcomes. Although both total and fetal DNA levels were always higher in women carrying pregnancies with chromosomal aberrations or having other pregnancy complications (P-values range from 0.028 to 0.267), these differences reached statistical significance only for total DNA levels between the group A and corresponding normal pregnancies (P = 0.028). The correlation between the fetal/total DNA ratio obtained by real-time PCR and the percentage of relative fetal allele concentration in maternal alleles obtained by QF-PCR was not found to be statistically significant (r(2) = 0.04; P = 0.76).

CONCLUSION

Our results confirm the clinical value of fetal DNA measurement in maternal plasma during the second trimester as a supplement for the diagnosis of aneuploidies. Its use as a screening instrument for complications that develop later in pregnancy seems to be limited but needs further investigation. Although the QF-PCR assay has the advantage of being applicable to both female and male fetuses, this approach cannot be used for quantitation of cff DNA in maternal plasma samples.