Scientists have found a way to postpone labour which they hope could eventually be used to prevent premature births.
Prematurity is a risk
At present the technique has only been tested in mice - but they say there is no reason why it should not also work in humans.
A team from the University of Texas, US, were able to prevent mice from going into labour for an extra one or two days - highly significant as the animal's average length of gestation is only 19 days.
Throughout pregnancy the uterus is prevented from contracting by a hormone called progesterone.
However, at birth the hormone ceases to have the same inhibitory effect, and contractions begin.
The US team were able to extend the effect of progesterone by injecting mice with a drug called tricostatin A.
The drug acts on receptors which are stimulated by progesterone throughout pregnancy, but which are switched off around the time of labour.
Tricostatin A stops the off switch from being flicked - and enables progesterone to continue to inhibit labour for a longer period.
Lead researcher Professor Carole Mendelson said premature birth was a significant cause of baby death and illness.
She said: "Our findings could impact the care of pregnant women and the well-being of their babies in the future."
Mr Ron Lamont, a consultant obstetricians at Northwick Park Hospital, London, UK, told BBC News Online that a drug to prevent prematurity could be highly significant - provided it acted at a relatively early stage.
He said: "Between 22 and 26 weeks of pregnancy, each extra day that a pregnancy can be extended improves a baby's survival chances by 3%.
"However, if the drug only worked between weeks 32 and 36, for instance, then it is not going to have very much impact at all. A baby's survival chance at 30 weeks is already 90%."
The research is published in the journal Proceedings of the National Academy of Sciences.