Preliminary results, published in the journal Science, suggest it is accurate in up to 80% of high-risk women.
The team, at Stanford University, in the US, say it is also as accurate as ultra-sound scans at predicting due dates.
However, there is still far more work to do before it could be used clinically.
Every year 15 million babies are born too early (before 37 weeks gestation) around the world
Preterm birth is linked to a million deaths a year
It is the leading cause of deaths among children under the age of five
The test measures the activity of genetic material, called RNA, coming from the foetus, placenta and mother that ends up in the bloodstream.
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.
The blood test was accurate 45% of the time at predicting gestational age in experiments involving 38 women, compared with 48% for ultrasounds, the researchers say.