The pessary is designed to strengthen the cervix -- the lower end of the uterus that leads to the vagina -- so that it can cope with the extra weight of the final weeks of pregnancy.
Silicon pessaries have been used over the past 50 years as one of several methods to prevent pre-term births.
But their effectiveness has been debated, and this was the first time the device had been investigated in a randomized trial.
Six percent of women who were fitted with the pessary gave birth prematurely, compared to 27% of counterparts who did not have the device, according to the study.
The so-called PECEP trial recruited 15,000 women who underwent ultrasound examination in five hospitals when they were in between 20 and 23 weeks of pregnancy.
Of these, 380 had cervical shortening -- defined as a having a cervix whose length was 25mm (0.98 inches) or less -- and were randomly assigned to one of two groups, each comprising 190 women.
In the pessary group, 12 had a baby before 34 weeks of pregnancy, while the number in the non-pessary group was 51.
No side effects were reported in the pessary group, and 95% of its participants said they would recommend the treatment for others.
"Placement of a pessary is an affordable procedure, non-invasive and easy to insert and remove as required," said lead researcher Maria Goya, an obstetrician at the Vall d'Hebron University Hospital in Barcelona. - (www.rappler.com)