The main objective of this study is to demonstrate that antenatal corticosteroids administration during a spontaneous onset of labor betwen 35 0/7 and 36 6/7 of gestation reduces respiratory distress in the late preterm infants. Design: Randomized triple blind clinical trial. Setting: Multicenter study. The study will involve 5 centers, wich Hospital Universitari Josep Trueta (Girona) will be the reference center. Participants: Pregnant woman with spontaneous onset of labor between 35 0/7 and 36 6/7 of gestation. Interventions: 24mg of betamethasone or placebo intramuscularly in two doses separated by 24h. In order to that betamethasone has enough time to take effect on the fetus, a pattern of Atosiban, a tocolytic to delay the uterine dynamic will be administered. Main outcomes measures: Primary outcome will be the indicence of respiratory distress in the late preterm infant. Secondary outcomes will include other complications in the late preterm infants