The thymic shadow is often visible on the chest radiographs of preterm infants. However thymic size is known to respond to stresses such as infection, endogenous or exogenous steroids. We selected two groups of infants retrospectively in order to investigate whether the thymus responds to the antenatal steroid routinely given to mothers in premature labour. Groups of 20 infants of 26-32 weeks gestation were matched for sex, birthweight (± 100g) and gestation (±1 week). Group 1 was born during 1992-1993 (no antenatal steroid) and group 2 during 1995/6 (all mothers received two doses of antenatal steroid). In all infants the initial chest radiograph, taken within the first 36 hours, was examined. In group 1 we observed thymic shadows in 45% of the CXRs, in group 2 none showed a thymic shadow (p <0.001). In 2 of the group 2 infants we observed a steady enlargement of the thymic shadow on the chest radiograph over the first 4 weeks of life. Many of the remainder did not have a clear thymic shadow on CXR at two months, but thymic tissue could be clearly seen using ultrasound. In both groups of infants the total white cell counts and lymphocyte counts remained within the normal ranges. Mothers of group 1 infants with no thymic shadows predominantly had hypertension or prolonged rupture of membranes. Antenatal steroid therefore significantly reduces the size of the infant thymus; this reduction is reversible and appears to have no effect on the total lymphocyte count in the infants studied.
