A team from Brighton and Sussex University Hospitals reviewed seven studies of 297 babies.

Around half of units in the UK do wait, but others cut the cord as quickly as 10 or 15 seconds after birth.

The review is published by healthcare information group, the Cochrane Collaboration.

The seven studies which were reviewed measured blood pressure, red blood cell counts, blood volume, bleeding within the brain and the need for transfusions.

Between 60 and 80% of preterm infants less than 32 completed weeks' gestation require transfusion.

But premature babies often have trouble breathing, so doctors aim to move them to special care baby units where they are helped to breathe, which requires the umbilical cord to be clamped and cut quickly.

'A healthier start'

Medical staff ordinarily clamp the umbilical cord in two places after the baby is delivered, then cut the cord between the two clamps.

There are no formal guidelines for when the cord should be cut. The latest evidence showed 47% of units performed delayed cord clamping - anything between 30 seconds and two minutes after birth,

The researchers say reducing the chances of bleeding in the newborn's brain also cuts the need for transfusions.

They found the delay also reduces anaemia and increases blood pressure and blood volume, giving premature infants a healthier start in life.

Dr Heike Rabe, the neonatologist who carried out the review told BBC News Online: "A slight delay in cord clamping of preterm infants is good for their subsequent health.

"It is cheap, leading to no extra cost. The optimal timing is not known yet and needs to be assessed by further studies. Funding needs to be available to perform this clinical research."

She added: "If the cord is left unclamped for a short time after the birth, some of the baby's blood from the placenta passes to the baby to help the flow of blood to the baby's lungs," Rabe explains. "Delaying cord clamping for just a very short time helped the babies to adjust to their new surroundings better."

However, other doctors say it is not possible to set down a rule which would be applied to all babies.

'Thickening the blood'

Dr Tonse Raju, a neonatologist at the US National Institute of Child Health and Development in Bethesda, Maryland, said: "I'm comfortable with the 30-second delay, but there are so many things that can happen with a preterm infant that doctors have to use their judgment in each case.

"Blood pressure in preterm infants is so narrow that even seconds can make an important difference."

Dr Raju added: "A little extra blood can help restore blood pressure."

Low blood pressure may require transfusions of blood or fluids, which can be tricky to accomplish safely in a preterm baby.

Dr Raju said delaying clamping too long can send too many red blood cells into the baby's system. That can make the blood too thick, stressing the heart and respiration, and possibly triggering jaundice or brain damage.

Such a delay may also prevent adequate resuscitation or unnecessarily expose the baby to cold.

However, Dr Rabe said that despite concerns for the baby's respiratory status, the trials covered in the review offered little guidance about how breathing is affected by cord clamping time.

"At least there was no negative effect on babies' breathing after delaying the clamping of the cord."