A simple ring can help reduce the risk of premature birth in some women, a Spanish study suggests.

Researchers tested the cheap device, called a pessary, in women who had a short cervix in the lower part of the womb.

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Pregnant women have their cervix measured with ultrasound. (Joshua Lott/Reuters)

A full term pregnancy is when a baby is born at 37 weeks or later. Preterm birth increases the risks of many health problems for the baby.

Doctors focus on risk factors for premature labour such as managing diabetes and high blood pressure in women but there are few proven ways to safely prevent early labour.

It's known that having a cervix that measures shorter than 25 millimetres during routine second trimester ultrasound increases the risk of premature labour.

In the study published in this week's issue of the medical journal The Lancet, researchers randomly assigned 385 women aged 18 to 42 with cervical shortening to receive a pessary, or to watchful waiting that steps in with treatment only after a medical need arises.

About five women with a short cervix would need to be treated with the device to prevent one baby from being born before 34 weeks, the researchers estimated.

The researchers did not find any serious adverse events associated with the cervical pessary.

"Cervical pessary use could prevent preterm birth in a population of appropriately selected at-risk women previously screened for cervical length assessment at the midtrimester scan," Maria Goya of Vall d'Hebron Hospital in Barcelona and her co-authors concluded.

How many benefit?

The National Health Service in the UK described the research as "exciting" in calling for the findings to be confirmed in larger studies in other countries.

It is difficult to predict which women are likely to have a preterm birth. Only six per cent of the women who were initially screened in the study met the criteria so the agency said it's impossible to tell if most pregnant women would benefit from a pessary.

The need to suppress labour using drugs or to give the newborn corticosteroid treatment to try to mature the lungs after birth was higher in the control group than among those born to women with a pessary.

Inserting and removing the pessary was ranked as painful by participants but 95 per cent of the women who received it said they'd recommend it to others in the same category, the NHS said.

While pessaries have been used for over 50 years to prevent preterm birth, there wasn't strong evidence backing their use. Today the devices are made of silicone.

The study was not large enough to tell how well the babies fared. The researchers are planning to study the infants until age two to look for any developmental differences between the two groups.

A pessary costs about $50.

The study was funded by Instituto Carlos III.