A Cochrane review included five studies with a total of 344 twin pregnancies. All trials compared oral betamimetics to placebo.
Betamimetics reduced the incidence of preterm labour (one trial, 50 twin pregnancies, relative risk (RR) 0.40; 95% confidence interval (CI) 0.19 to 0.86). However, betamimetics did not reduce preterm birth less than 37 weeks' gestation (four trials, 276 twin pregnancies, RR 0.85; 95% CI 0.65 to 1.10) or less than 34 weeks' gestation (one trial, 144 twin pregnancies, RR 0.47; 95% CI 0.15 to 1.50). Mean neonatal birthweight in the betamimetic group was significantly higher than in the placebo group (three trials, 478 neonates, weighted mean difference 111.2 grams; 95% CI 22.2 to 200.2).
There was no evidence of an effect of betamimetics in reduction of low birthweight (two trials, 366 neonates, RR 1.19; 95% CI 0.77 to 1.85) or small-for-gestational age neonates (two trials, 178 neonates, RR 0.92; 95% CI 0.52 to 1.65). Two trials (388 neonates) showed that betamimetics significantly reduced the incidence of respiratory distress syndrome but the difference was not significant when the analysis was adjusted for correlation of babies from twins. Three trials (452 neonates) showed no evidence of an effect of betamimetics in reducing neonatal mortality (RR 0.80; 95% CI 0.35 to 1.82).