Researchers have developed a new blood test for pregnant women that may detect whether their pregnancies will end in premature birth.

The technique can also be used to estimate a foetus's gestational age -- or the mother's due date -- as reliably as and less expensively than ultrasound, the researchers said. "This work is the result of a fantastic collaboration between researchers around the world," said co-author Stephen Quake, Professor at the Stanford University in US.

The findings, published in the journal Science, suggested that the tests could help reduce problems related to premature birth, which affects 15 million infants worldwide each year. According to the researchers, until now, doctors have lacked a reliable way to predict whether pregnancies will end prematurely, and have struggled to accurately predict delivery dates for all types of pregnancies.

The tests measure the activity of maternal, placental and foetal genes by assessing maternal blood levels of cell-free RNA, tiny bits of the messenger molecule that carry the body's genetic instructions to its protein-making factories. For the study, the team used blood samples collected during pregnancy to identify which genes gave reliable signals about gestational age and prematurity risk.

The gestational-age test was developed by studying a cohort of 31 women who gave blood weekly throughout their full-term  pregnancies. The scientists used blood samples from 21 of them to build a statistical model, which identified nine cell-free RNAs produced by the placenta that predict gestational age, and validated the model using samples from the remaining 10 women.

The estimates of gestational age given by the model were accurate about 45 per cent of the time, which is comparable to 48 per cent accuracy for first-trimester ultrasound estimates. To figure out how to predict pre-term birth, the researchers used blood samples from 38 women who were at risk for premature delivery.

These women each gave a blood sample during the second or third trimester of their pregnancies. Of this group, 13 delivered prematurely, and the remaining 25 delivered at term. The scientists found that levels of cell-free RNA from seven genes from the mother and the placenta could predict which pregnancies would end early. The new tests however need to be validated in larger cohorts of pregnant women before they can be made available for widespread use, the researchers said.