A common surgical procedure used to try to stop women giving birth prematurely has little effect, research suggests.
Premature birth was defined as delivery at or before 33 weeks
Cervical cerclage involves the use of stitches to keep the cervix closed.
A team from Kings College Hospital London identified women with short cervixes who are at increased risk of preterm birth.
Their study, in The Lancet, found that, for this group, cervical cerclage cut the odds of preterm birth by only a tiny amount.
Despite this, the technique has been widely used for 50 years to prevent early preterm birth, which is associated with higher rates of death and illness among babies.
The death rate rises from about 2% for babies born at 32 weeks, to more than 90% for those born at 23 weeks.
About six out of 10 babies who survive being born at 26 weeks are disabled.
The Kings team, led by Professor Kypros Nicolaides, used ultrasound screening to identify 250 women with a short cervix. Some underwent cervical cerclage, while others had no surgery.
Among those who underwent surgery, 22% went on to have a premature birth, compared to 26% among those who had no surgery.
Premature birth was defined as delivery at or before 33 weeks.
In both cases, the level of premature birth was far higher than the UK average of 1.5%.
The researchers say their work shows that women with short cervixes are at increased risk of premature birth - and that for them cervical cerclage seems to have only a limited effect.
Writing in The Lancet, Professor Nicolaides' team said that they initially felt that the introduction of routine ultrasound and cervical cerclage could only be justified if it led to a three-fold reduction in premature births.
Cervical cerclage does carry an increased risk of infection, and going into labour with the stitching still in place can cause tearing.
However, they go on to say: "We acknowledge that a more modest reduction in early preterm delivery might be beneficial because of the enormity in both the human and economic cost of prematurity."
Charlotte Davies, from Tommy's, the baby charity, told BBC News Online that prematurity had "huge implications both in emotional terms for families and in the long-term health of the baby".
She said: "It is important that the research team have identified that trans-vaginal ultrasound identifies women at increased risk of preterm delivery as the more accurate identification of these women means that more can be done to care for them during their pregnancy."