The test estimated due dates within two weeks in almost half the cases, making it as accurate as the current, more expensive method, ultrasound, and more accurate than guesses based on a woman's last period.
Scientists are trialling a blood test that may predict whether a pregnant woman will give birth prematurely.
"It's mostly maternal genes", says Stanford University medical researcher Mira Moufarrej.
Researchers have developed an low-priced blood test they say can predict a pregnant women's due date and potentially identify women at risk for of preterm birth; Virginia's governor has expanded Medicaid in the state; and the American Medical Association is expected to take a stance on over-the-counter birth control this weekend.
In two separate cohorts of women, all at elevated risk of delivering preterm, the research team identified a set of cell-free RNA (cfRNA) transcripts that accurately classified women who delivered preterm up to two months in advance of labor. If much larger studies achieve comparable results, the test could become a tool to help prevent unnecessary induction of labor or cesarean deliveries, and could possibly help save babies who would have died because they were born too early.
Prof Basky Thilaganathan, a Royal College of Obstetricians and Gynaecologists spokesman, said: "Complications from premature birth are a leading cause of infant mortality and affect 7-8% of all births in the UK".
It is estimated that around 15m babies are born early across the world each year, equating to more than one in 10 births, with more than 1m of those dying shortly after.
"I've spent a lot of time over the years working to understand preterm delivery", Mads Melbye, senior author of the study and the president and CEO of the Statens Serum Institute in Copenhagen, said in a statement.
A newly developed blood test can predict the gestational age and preterm birth risks of expecting mothers.
The tests measure the activity of maternal, placental and foetal genes by assessing maternal blood levels.
Currently, age is predicted using ultrasound imaging, which is expensive and inaccurate beyond the first trimester.
The researchers started by taking blood samples from pregnant women every week to see how levels of different RNAs changed during pregnancy and which could be used to predict gestational age or a premature birth.
When will a pregnant woman actually deliver her baby? Existing tests at best can provide a premature delivery to women at high risk, who have already given birth prematurely, but again their accuracy is only around 20%.
"The strength of the study is showing there are molecular milestones that are achieved by the fetus and by the placenta", she said.
The new test tracks what's happening in pregnancy via genetic clues in the mother's blood.
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".