May 2, 2012 -- Researchers say they've found a way to identify the father of a baby as early as the eighth week of pregnancy.

The new method, reported in the New England Journal of Medicine, uses fetal DNA isolated from the mother's blood.

The test has already been used to prove an affair in a murder trial, and experts say it has other significant medical and legal applications.

"The technology is extremely important and really a very, very significant step forward for medical prenatal diagnosis," says Peter Benn, PhD, director of the diagnostic medical genetics laboratories at the University of Connecticut Health Center in Farmington. He was not involved in the research.

A previous study by the same researchers showed how the test could be used early in pregnancy to determine whether a fetus has Down syndrome, for example.

"That technology will allow genetic disorders to be identified without going through amniocentesis or chorionic villus sampling. That's a very important and useful step forward because those tests are not without some degree of risk to the fetus," Benn says.

Amniocentesis, in which a small sample of fluid that surrounds the fetus is removed and analyzed, and chorionic villus sampling, in which a small piece of tissue from the placenta is removed and analyzed, can cause infections, miscarriages, and birth defects. These tests are not able to be done before 10 to 15 weeks of pregnancy.

"Before we could only rely on tests that were invasive and could only be done later in pregnancy," says Jill Rabin, MD, chief of ambulatory care, obstetrics, and gynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y.

"Here you have a blood test with the possibility of giving you an answer so that a woman can decide whether to proceed with a pregnancy or not based on the results of a noninvasive test. That is, right there, a tremendous utility," says Rabin, who was not involved in the research. "This is really cutting-edge stuff. It's very exciting."