A popular hormone treatment given to pregnant women to stop them having their babies too early does not work and is a waste of time, a new study has found.

A study of 789 women in New Zealand, Australia, Canada found giving progesterone pessaries to women with a history of pre-term birth made no difference compared to the ones who did not use them.

Lead researcher Professor Caroline Crowther, from Auckland University's Liggins Institute, said there had previously been mixed studies about whether the use of vaginal progesterone reduced preterm birth, Because there was no definitive answer, treatments given to women with a history of having premature babies varied widely among hospitals and countries, she said.

Auckland University professor Caroline Crowther led research which found using progesterone devices to prevent premature births was a myth. Photo/Supplied
Auckland University professor Caroline Crowther led research which found using progesterone devices to prevent premature births was a myth. Photo/Supplied

Progesterone pessaries can be prescribed to woman who have given birth before 37 weeks as there was evidence that it could delay birth and therefore the prevent the risks associated with a premature baby such as breathing problems or other serious health issues.

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As part of the study half the women - being treated at 39 maternity hospitals across the three countries - were given the progesterone treatment from 14 weeks and the other half were given a placebo.

The mothers and babies then assessed the health of the mothers and babies and said it was similar among those who had progesterone and those who had not, proving taking progesterone was a waste of time.

"We now have a clear answer we can believe: vaginal progesterone does not improve the baby's health when given to women who have had a history of a previous preterm birth," Crowther said.

"This is good for pregnant women and health practitioners to know, so they won't waste time and resources on an ineffective treatment."