Babies' heart rates were monitored
Electromagnetic fields from incubators may be interfering with newborn babies' heart rates, claim researchers.
The Archives of Disease in Childhood study found normal changes in heart rate were reduced when the machines were turned on.
However, the Italian researchers found no hard evidence of any actual health damage caused by incubators.
The Medicines and Healthcare Products Regulatory Authority has received no reports of problems with the equipment.
Many thousands of newborns need an incubator to help keep them healthy, some for many months after birth.
The main job of the machine is to keep the air surrounding the baby warm.
But using its motor creates an electromagnetic field in the area immediately surrounding it, which covers the area where the baby lies.
To see whether this had any effect on the infant, the researchers from the University of Siena looked at "heart rate variability", the natural switching up and down of the heart rate by the body.
These changes are thought to be a good thing - in adult patients with heart disease, decreased heart rate variability can be used by doctors to predict a worse outcome.
There is no evidence, however, suggesting the same is true of babies.
A total of 27 babies, none of whom actually required incubator care, were assessed over three periods of five minutes each, during which the incubator motor was left running, then switched off, then left running again.
During the "switched on" periods, heart rate variability in the babies fell significantly.
The researchers checked to make sure it was not simply the noise of the motor causing the effect by playing a tape recording instead, but no effect was found.
The researchers made it clear that it was unclear whether there were any actual health consequences from exposure to electromagnetic fields at a young age, but said that modifications to incubator design might be considered.
"International recommendations and laws set levels to safeguard the health of workers exposed to electromagnetic fields - newborns should be worthy of similar protection," they wrote.
Dr Carlo Bellieni, who led the study, said that he did not want to alarm parents, but that a precautionary approach was necessary.
"We know that this heart rate variability has been linked to arrythmias and strokes in adults, but we do not know yet the consequences of it for these tiny babies.
"What we have proved is that the effects of these machines are not neutral - and they should be."
"More research into the results of this exposure must take place, and the manufacturers of these incubators should take steps to shield the babies from their motors, and move the motors further away within the machine."
In the UK, the safety of medical devices such as incubators falls under the remit of the Medicines and Healthcare Products Regulatory Authority, which said it was "unaware" of any problem.
A spokesman said: "The MHRA has had no reports of the electromagnetic fields in incubators affecting the heart rates of newborn babies.
"If it was shown that these could present a hazard we would alert users to the problem and give suitable advice, as well as assess the design and manufacture of the incubators."