Choice of primary outcomes in randomised trials and systematic reviews evaluating interventions for preterm birth...

The inappropriate and inconsistent selection of primary outcomes (POs) in randomised controlled trials (RCTs) and systematic reviews (SRs) can make evidence difficult to interpret, limiting its usefulness to inform clinical practice.

To systematically review the choice and consistency of POs in RCTs and SRs of preventative interventions for preterm birth.

Cochrane Pregnancy and Childbirth Group's Specialised Register of trials and a full list of published reviews and protocols.

Full reports of RCTs for preterm birth prevention published after CONSORT (January 1997-January 2011), and Cochrane Reviews and protocols relevant to preterm birth prevention, for the same period.

For RCTs, the PO was the outcome used for sample size calculation. For SRs, we included all outcomes listed as 'primary'. Two review authors selected studies and double-checked the data for accuracy.

Seventy-two different POs were reported by 103 RCTs. The three most common POs were based on length of gestation, with preterm birth before 37 weeks of gestation being the most common (18/103, 18%). Few RCTs chose perinatal morbidity (4/103) or mortality (1/103), or their composites (5/103), as POs. In 33 Cochrane Reviews, 29 different POs were reported. The three most common POs were based on death or morbidity in the baby, with death of the baby being the most common (22/33, 67%). POs were variably defined.

There is a lack of consistency in the choice and definitions of POs in clinical research related to preterm birth prevention. SRs are more likely to report morbidity and mortality as POs, whereas RCTs tend to use length of gestation. Researchers are urged to review the outcomes reported in RCTs and SRs in their respective areas of interest to highlight discrepancies and facilitate the development of core outcome sets.