Postponing delivery by a few days may prevent brain damage in premature babies, a study suggests.
Delay delivery for as long as possible, say the researchers
Doctors know allowing the baby to mature in the womb improves the chance of survival and reduces the risk of prematurity and cerebral palsy.
But delaying delivery of a baby small for its age that is not growing well because of a lack of oxygen in the womb can also cause brain damage.
On balance, deferring delivery was best the Lancet study found.
According to the Nottingham University researchers who carried out the investigations funded by the Medical Research Council (MRC), theirs is the first study to assess the impact of different delivery times for premature babies on survival and brain development.
Dr Jim Thornton and colleagues studied 548 pregnant women from 69 hospitals in 13 European countries.
Each woman's pregnancy was between 24 and 36 weeks gestation and their unborn babies were smaller than is usual for that particular stage of pregnancy.
It is arguable that promptly delivering these stressed babies with stunted growth is beneficial because it removes them from the hostile environment of the womb in which they are failing to grow.
On the flip side, these babies can also be immature and delaying their delivery can allow them to develop further in the womb.
The women in the study were randomly assigned to either have their labour induced immediately or to have their delivery delayed for as long as possible - around four days on average.
The researchers then assessed the outcomes of the 588 babies born to the women.
There were no differences in the baby death rate between the two groups or in the overall brain development assessed when the children were two years old.
Delay prevented damage
However, the rate of disability - mainly cerebral palsy - was slightly higher in the group of babies who had been delivered immediately.
Overall, 21 of these 290 babies were disabled compared with 12 out of the 283 babies in the deferred delivery group.
The researchers said further research would be needed to confirm this trend because the difference was only small.
In the meantime, they said it would be best to err on the side of caution and delay delivery where possible.
Dr Thornton, professor of obstetrics and gynaecology at Nottingham City Hospital, said: "There has to be an element of best judgement in these situations and hopefully our results will give doctors better information to make those crucial decisions.
"The lack in difference in mortality suggests that obstetricians are delivering sick preterm babies at about the correct moment."
But he said the findings should discourage doctors who deliver premature babies before the point at which delivery could be delayed no longer.
"This caution applies particularly to pregnancies for which early delivery is considered before 30 weeks. In this situation, we believe that the obstetrician should delay," he said.
The researchers will follow up the same children until they are nine years old to check that the timing does not cause any problems later in life.
Mr David Liu, consultant obstetrician and gynaecologist and clinical directory at the City Hospital in Nottingham said: "Although there is some hint that there's some improvement one has to be a little bit circumspect about the outcome really because the delays are only a few days. It's not a very long delay."
He said this questioned what was causing the apparent benefit when you keep a baby whose growth is restricted by it's environment in the same environment for longer.
"Are we talking neurological damage? Are we asking the right questions?
"The immature baby would have some benefit. It's a balance."
A spokeswoman from the premature baby charity BLISS said: "Any practice to reduce this risk is seen as positive news.
"While we appreciate the dilemma faced by doctors when determining when to deliver babies at risk, we hope the results of this study and further research may contribute to what seems to be a positive change in practice."