Possibly the observation that has changed future prospects for the noninvasive diagnosis of fetal genetic traits the most was that describing the presence of extracellular fetal DNA in maternal plasma and serum. This analysis was prompted by reports indicating that cell-free tumor-derived DNA was readily detectable in the plasma of cancer patients. By arguing that the placenta shares many similarities with tumors (high rate of cell turnover, tissue invasion), it was hypothesized that placenta-derived cell-free fetal DNA may be present in the maternal circulation. This indeed turned out to be the instance, with cell-free fetal DNA being readily detectable by conventional PCR assays. However, a caveat of this fetal genetic material is that, because cell-free maternal DNA is also present in the maternal circulation, only paternally inherited loci absent from the maternal genome can be reliably detected. Consequently, most studies have focussed on the detection of such facile fetal genetic loci such as Y chromosome-specific sequences.
An advantage of cell-free fetal DNA is that the concentration of this material in maternal samples can be reliably quantified by the use of real-time PCR. Studies employing this technology indicated that this analyte had a very short half-life (on the order of 15 min) and rapidly disappeared from the maternal periphery postdelivery. It was also shown that cell-free fetal DNA constitutes approximately 5% of the total circulatory cell-free DNA, and that the number of copies of this fetal material was approximately 100-fold higher than the estimated number of circulatory fetal cells. Current consensus is that cellfree fetal DNA is almost exclusively derived from the placenta.
Furthermore, by the use of such quantitative PCR technology, it was clearly demonstrated that the concentrations of this fetal analyte were elevated in pregnancies with aneuploid fetuses (again most notably with trisomy 21),[12,13] or those affected by preterm labor and preeclampsia.[14-16] A valuable aspect of these studies is that, by being performed independently, they verified the soundness of the initial observation and, for the first time, introduced a high degree of reproducibility into this research arena.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.