Objectives To determine the effectiveness of corticosteroids in reducing respiratory disorders in infants born at 34-36 weeks’ gestation.

Design Randomised triple blind clinical trial.

Setting A large tertiary teaching hospital in northeast of Brazil.

Participants Women at 34-36 weeks of pregnancy at risk of imminent premature delivery.

Interventions Betamethasone 12 mg or placebo intramuscularly for two consecutive days.

Main outcomes measures Primary outcome was the incidence of respiratory disorders (respiratory distress syndrome and transient tachypnoea of the newborn). Secondary outcomes included the need for ventilatory support, neonatal morbidity, and duration of stay in hospital.

Results 320 women were randomised, 163 of whom were assigned to the treatment group and 157 to the controls. Final analysis included 143 and 130 infants, respectively. The rate of respiratory distress syndrome was low (two (1.4%) in the corticosteroid group; one (0.8%) in the placebo group; P=0.54), while the rate of transient tachypnoea was high in both groups (34 (24%) v 29 (22%); P=0.77). There was no reduction in the risk of respiratory morbidity with corticosteroid use even after adjustment for subgroups of gestational age (34-34⁺⁶ weeks, 35-35⁺⁶ weeks, and ≥36 weeks). The adjusted risk of respiratory morbidity was 1.12 (95% confidence interval 0.74 to 1.70). The need for ventilatory support was around 20% in both groups. There was no difference in neonatal morbidity (88 (62%) v 93 (72%); P=0.08) or in the duration of stay in hospital between the two groups (5.12 v 5.22 days; P=0.87). Phototherapy for jaundice was required less often in babies whose mothers received corticosteroids (risk ratio 0.63, 0.44 to 0.91).

Conclusions Antenatal treatment with corticosteroids at 34-36 weeks of pregnancy does not reduce the incidence of respiratory disorders in newborn infants.

Trial registration Clinical Trials NCT00675246.