Sucking clear a baby's airways to prevent a potentially fatal respiratory disease is probably ineffective, say researchers.
Inhaling faeces during delivery can be dangerous
Around 10% of babies are at risk of inhaling their own faeces during labour, which may put them at risk of respiratory problems.
However, doctors have found the widely practice procedure of 'suctioning' the airways does little to cut the risk.
The University of Buenos Aires research is published in The Lancet.
The researchers say that although suctioning has been widely practised since the 1970s, there has been little hard evidence of its usefulness.
Doctors tend to use it as a precaution when there are signs that the amniotic fluid surrounding a baby has been stained with the baby's faeces - known technically as meconium.
If meconium, which is very sticky, gets down into a baby's lungs, it can lead to tissue damage and a serious respiratory problem known as meconium aspiration syndrome.
Doctors hope that by making sure the airways are clear after birth the risk of this happening will be minimised. However, the procedure itself is not without risk.
It is usually carried out when only the baby's head has been delivered, as the infant does not take its first breath until its body has been delivered as well.
The Buenos Aires team, lead by Dr Nestor Vain, carried out a trial on 2,500 babies. Some received suctioning at the mouth and nose, and others no suctioning at all during delivery.
The researchers found around 4% of babies in each group developed symptoms of meconium aspiration syndrome.
Around 1%-2% of babies in each group required mechanical ventilation.
Dr Vain said: "We conclude that routine intrapartum oropharyngeal (mouth) and nasopharyngeal (nose) suctioning of term-gestation, meconium-stained infants does not prevent MAS or its complications.
"There are many ongoing malpractice lawsuits based on the mistaken belief that if a baby develops respiratory distress associated with meconium aspiration the responsibilities belong to the obstetrician and neonatologist who failed to appropriately clear the airway.
"We hope that our study will bring light to this erroneous belief."
Dr Vain told BBC News Online that respiratory disorders were more likely to be linked to longer term problems involving constriction of the vessels taking blood to the lungs to pick up oxygen, and not to airway obstructions caused by taking in meconium.
Ms Maggie Blott, an obstetrician at King's College Hospital, agreed that suctioning the airways probably did not prevent meconium aspiration syndrome.
However, she it was common practice to carry out the procedure on babies who had inhaled meconium simply to ensure there was no obstruction in their airways, and that they were breathing easily.
Dr Peter Bowen-Simpkins, of the Royal College of Obstetricians and Gynecologists, told BBC News Online that suctioning tended to be carried out as a matter of course by midwives, but paediatricians were more aware of its limited value.
He said: "We have accepted a huge amount of unproven therapies over the years, and as time goes on we are finding more and more of them turn out not to be of proven value."
Professor Stephen Robson, an expert in obstetrics and gynaecology at Newcastle Royal Infirmary, said he was not surprised by the findings.
He said: "The value of suctioning at the time of delivery has been questioned for some time.
"This reflects the fact some aspiration occurs before birth of head, and the inefficiency of oropharyngeal and nasopharyngeal suction."
But Dr Andrew Lyon, a consultant neonatologist at the Simpson Centre for Reproductive Health, Edinburgh, said: "This is a well run study that tells us that an accepted practice may well not be as necessary we might have thought."
However, Dr Lyon said the risk associated with the procedure may have been over-stated.