Dr. Quake, who invented the first noninvasive prenatal blood test for Down syndrome, said the relevant genes for gestational age were in the placenta, and the test’s predictions were most reliable in the second and third trimester.

The researchers then applied the test to two groups of women at risk for preterm birth — patients at the University of Pennsylvania who had premature contractions and patients at the University of Alabama at Birmingham who had delivered prematurely in a previous pregnancy. The gestational age blood test did not do a great job of predicting which women would deliver prematurely, suggesting those particular genes “may not account for the various outlier physiological events that may lead to preterm birth,” the study said.

But in analyzing the blood of some of the women who delivered prematurely, the team identified seven other genes, mostly from the women, with RNA signals that seemed to characterize preterm birth. Dr. Quake said the team is developing plans for a large clinical trial in the general population.

Dr. Diana Bianchi, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development and an expert in prenatal genetic testing, said a trial should involve as much diversity as possible. She noted that the genes relevant for estimating gestational age were identified using healthy Caucasian Danish women with full-term pregnancies. The women in the preterm birth analysis were African-American, and they represented only two of many potential risk categories for premature delivery, which can also be caused by infection, inflammation, maternal stress and other factors, she said.

“The strength of the study is showing there are molecular milestones that are achieved by the fetus and by the placenta,” she said. And while the blood test is unlikely to replace ultrasound, which provides other important information, “I think it will add another dimension to identifying high-risk pregnancies.”

To Dr. Edith Cheng, a professor of maternal-fetal medicine and medical genetics at the University of Washington, the study’s real significance is that it “cracked the door a little bit into potentially finding out how a fetus talks to the mother, what makes a pregnancy go.”

It is almost as though the molecular message being sent by RNA “is a little bus that travels back and forth and is letting Mom know what’s going on,” Dr. Cheng said. “I bet you they’re going to find that the mother’s going to respond. There’s a conversation going on. That’s what’s cool.”