Full versus half dose of antenatal betamethasone to prevent severe neonatal respiratory distress syndrome associated with preterm birth: study protocol for a randomised, multicenter, double blind, placebo-controlled, non-inferiority trial (BETADOSE) - Schmitz, T; Alberti, C; Ursino, M; Baud, O; Aupiais, C; BETADOSE study group and the GROG (Groupe de Recherche en Gynécologie Obstétrique) (BMC Pregnancy Childbirth, 2019)