Fetal DNA in maternal blood allows less invasive diagnosis of anomalies. (Tests in development).

OSLO -- Analysis of fetal DNA circulating in a mother's bloodstream may provide easier diagnosis of prenatal anomalies than fetal-cell testing, Dr. Diana W Bianchi said at the 18th European Congress of Perinatal Medicine.

Because it is easier for "naked" or cell-free fetal DNA to enter a pregnant woman's bloodstream, the DNA is available in much higher amounts. "Fetal cells are rare in a maternal blood sample, about one nucleated fetal cell per mL," said Dr. Bianchi, chief of perinatal genetics, department of ob.gyn., TuftsNew England Medical Center, Boston.

Fetal DNA tests are still in development in the United States but are approved in Europe. Although fetal cell testing allows specific diagnosis of aneuploidy, "It is difficult to perceive as a routine test given the level of difficulty of detecting these fetal cells," she said.

The DNA can be detected as early as 5-7 weeks' gestation. The source is unknown. "We are hypothesizing that DNA in plasma is of placental origin, whereas DNA in amniotic fluid is of fetal origin," said Dr. Bianchi. "A fetus is bathed in DNA. There is about 100-200 times more fetal DNA in the amniotic fluid compared to maternal plasma, even before urination begins.

A major limitation to both fetal cell and fetal DNA detection is that they are only possible with a male fetus, because a Y chromosome makes them easy to distinguish from a mother's DNA or cells. The male cells have been found in maternal circulation even long after a pregnancy. In fact, Dr. Bianchi mentioned a case in which male cells were found in a woman who had given birth to only daughters; however, her history included an early miscarriage many years before.

In an NIH-sponsored fetal-cell study, 1,359 male singleton ...