Blood tests could one day replace ultrasounds as a cheaper and more efficient way to determine the age of a developing foetus.

By searching for evidence of genetic activity in the mother's blood, it could be possible to not only pin down a delivery date, but determine whether the baby is at risk of being born before it's ready.

An international team of researchers has demonstrated it's possible to find and identify fragments of RNA associated with a child's development while it's still in the womb through a cheap and relatively non-invasive procedure.

While we're still some way off a practical clinical test, early trials indicate this new method could one day replace the more expensive ultrasound as a method for gauging gestation, with the added bonus of providing warning signs of a premature birth.

"We found that a handful of genes are very highly predictive of which women are at risk for preterm delivery," says the study's senior author Mads Melbye, president and CEO of the Statens Serum Institute in Copenhagen.

"I've spent a lot of time over the years working to understand preterm delivery. This is the first real, significant scientific progress on this problem in a long time."

Nearly one in ten births in nations like the US are premature, risking complications that make it a leading cause of infant mortality. Getting a heads-up on its risks has the potential to save a lot of lives.

The research team conducted a pilot study on 31 healthy pregnant women, using 21 of them to create a statistical model based on nine transcripts of RNA from the mother's immune system, the infant's liver, and the placenta.

This allowed them to estimate the birth period for the remaining 10 mothers with an accuracy of 45 percent.

This mightn't sound all that great, but it does put into the same league as an ultrasound analysis.

A second study on volunteers determined to be at a greater risk of delivering prematurely looked for combinations of genes that were associated with pre-term delivery.

"It's mostly maternal genes," says Stanford University medical researcher Mira Moufarrej.

"We think it's mom sending a signal that she's ready to pull the ripcord."

Analysing blood from eight mothers who gave birth prematurely, the test was correct for six of them, misclassifying just one of the 26 full term deliveries.

The figures are impressive, though much larger and more diverse sample sizes are needed before we can be confident they demonstrate something truly powerful.

Doctors typically try to conduct an ultrasound during the first trimester, but not all mothers access early pre-natal care and later scans don't provide the same amount of information.

In contrast, the results indicate a diagnostic tool based on RNA transcripts would be more accurate during the second and third trimesters.

A blood test would also be simpler to carry out, cheaper, and potentially provide a wealth of details on the foetus's development.

"This gives a super-high resolution view of pregnancy and human development that no one's ever seen before," says lead author Thuy Ngo from Stanford University.

DNA doesn't tell the whole story behind gestational complications, with multiple environmental risk factors such as smoking and diet also playing an important role in pre-term births.

But eavesdropping on the genetic conversation between mother and child just might provide the vital clues needed to give a new person the best chance at starting life.

This research was published in Science.