US researchers have come up with a simple blood test that can predict the delivery date of pregnant women.

They say the technique may some day be able to identify women at risk of preterm birth.

The blood test looks for certain types of RNA in blood which signify different stages of development.

Doctors currently rely on ultrasound imaging and/or the mother’s estimate of her last menstrual period to predict the gestational age of a foetus, but the former can be expensive and the latter can be imprecise.

Inaccurate estimates can sometimes lead to unnecessary induction of labour and Caesarean sections, extended postnatal care and increased medical expenses.

Researcher Thuy Ngo and colleagues previously found that changes in both the mother and the foetus could be monitored by noninvasively measuring cell-free RNA (cfRNA) transcripts from foetal tissues in maternal blood.

In tests so far, they analysed blood samples from a cohort of women over the course of their pregnancies to identify cfRNA indicative of the foetuses’ gestational age.

The blood test they developed based on the data could predict the gestational age (where the birth occurred within 14 days of the estimated due date) with 45 per cent accuracy, which is comparable to estimates of gestational age and due date based on ultrasound, at 48 per cent.

In related work, the researchers compared cfRNAs in the blood of women who had healthy pregnancies with those in the blood of women who were known to be at elevated risk of preterm delivery. By assessing cfRNA transcripts of just seven genes, they accurately classified six of eight preterm cases and misclassified only one of 26 full-term cases.

While promising, the studies were small. The researchers say their next important step will be to assess the reliability of the tests in large, blinded clinical trials.