A therapy widely recommended in the UK, Europe and the US to stop babies from being born too soon is ineffective, research shows.
The treatment does not appear to pose any harm to mother or baby but has no effect on preventing an early birth, the findings reveal.
Researchers say that use of the medicine should be reviewed. They also call for a re-doubling of efforts to find alternative interventions to prevent premature births.
Previous research suggested that the therapy - a hormone called progesterone - may stop pregnant women from giving birth early but little was known about its long term effects.
This latest trial - involving more than 1200 women - is the largest to assess the effects of the treatment on women and the first to study its effects in babies born after the therapy.
Researchers focused on women who are considered to have a greater risk of premature delivery - either because they have previously delivered a baby early or have lost a baby late in pregnancy.
Around a half of the women were given progesterone and the others were given a dummy pill.
The team found that whilst the therapy appeared to be safe, it did not reduce the risk of premature delivery and offered no notable health benefits for mother or baby.
More than 64 hospitals from around the UK were involved in the research, which was led by the Tommy's Centre for Maternal and Fetal Health at the University of Edinburgh.
The study is published in The Lancet. It was funded by the Efficacy and Mechanism Evaluation (EME) Programme, a partnership between the Medical Research Council and National Institute for Health Research.
Professor Jane Norman, Director of the Tommy's Centre for Maternal and Fetal Health at the University of Edinburgh, said: "Babies born too early have a much greater risk of short-term and long-term health problems. We need to find new strategies that help mums carry their babies to term."