We have previously reported a lack of effectiveness of antenatal steroids in preventing RDS in multiple gestation infants. The Collaborative trial on Antenatal steroids reported low levels of steroid in twins compared to singletons and in Twin B compared to Twin A. Hypothesis: In multiple gestation pregnancies treated with prenatal steroids levels attained in the fetuses do not reach the critical level required to activate the glucocorticoid receptor mechanism.. Aim: To measure and compare the levels of dexamethasone in maternal and fetal blood in singleton and multiple pregnancies after prenatal administration. Methods: Expectant mothers in established labour at Womens College Hospital, a perinatal center in Toronto, were enrolled in this prospective study after informed consent was obtained. 6mg of dexamethasone was administered intravenously at varying intervals prior to delivery. Maternal and cord bloods were obtained shortly after delivery and dexamethasone levels measured in the plasma using a previously validated radioimmunoassay. Results: 25 mothers (12 singletons, 13 multiples) were enrolled in the study. There was no difference in mean gestation at delivery between the two groups (35 vs 36 wks- Range 24-40). In the twin group there was no difference in levels between Twin A and Twin B at all times measured (p=0.98, Pearson R=0.94). Levels in cord blood measured between 10 - 20 mins after administration of the steroid were not lower in twins compared to singletons but the exponential decline in levels over time was steeper for multiples and after 70 mins was well below that for singletons. Peak maternal levels were higher in multiple cf singleton pregnancies and the exponential decline similar in both groups. Conclusions: The results do not support the hypothesis that lower peak levels and differences in levels in twins account for the observed clinical differences in the effectiveness of antenatal steroids to prevent RDS in twins. We speculate that the half life of dexamethasone in the fetal compartment of twin pregnancies is much reduced probably due to increased metabolism and that a more frequent dosing regime may be needed in this group for optimal effectiveness.