A blood test can predict how far along a woman is in her pregnancy and also identifies women at risk of delivering their babies early.

Obstetricians determine when a pregnant woman is likely to give birth using either costly ultrasound scans or menstrual-cycle dates, which are often unreliable. In search of an alternative, Mads Melbye and Stephen Quake at Stanford University in California and their colleagues analysed cell-free RNA in the blood of 38 pregnant women. The team found that measurements of RNA from nine genes expressed in the placenta estimated gestational age with an accuracy comparable to that of an ultrasound scan. A similar blood test flagged women who would go into labour early.

The authors caution that a larger trial, including a more diverse cohort of women, is needed before the tests can be used in clinics.