Abstract 1241

The physiological stresses of delivery on the infant are a challenge to quantitiate. The thymic response to endogenous steroid has been used to estimate these factors. We measured thymic parameters weekly in infants on the postnatal ward or outpatients using an Aspen Acuson with a 7 Hz probe. Measurements of greatest thymic width, depth and length were collected by a blinded observer and multiplied to estimate thymic volume. All infants were healthy, and were receiving ultrasound scans for antenatally detected choroid plexus cysts or dilated renal pelvices. Two cohorts were studied: the first comprised 14 infants delivered by elective caesarian section at 38-40 weeks gestational age (median 39 weeks) to mothers with uncomplicated pregnancies. Their birthweights were 2.8 - 4.1 kg (median 3.6 kg). The second cohort of 10 infants was delivered vaginally without instrumentation at 37-41 weeks gestation (median 39 weeks) to mothers with uncomplicated pregnancies, following 6-13 hours in the second stage of labour (median 9 hours). All infants had Apgars of greater than 5 at a minute and none required admission to the neonatal unit. There were no statistically significant differences between thymic volumes at birth (p<0.09), but the rate of thymic growth was significantly greater (p<0.02) in those delivered vaginally over the first month of life. This suggested that vaginally delivered infants had sustained a greater reduction in thymic volume in early life, supporting the hypothesis that such infants are exposed to a greater degree of physiological stress and produce more endogenous steroid. All infants tested (6 in the first cohort, 7 in the second) had lymphocyte counts in the normal range, and no significant difference was detected between the two cohorts. Thymic perfusion estimated by Power Doppler was probably greater in the vaginally delivered group (the method requires refining), but reproducible measures of thymic size or circulation antenatally has proved technically difficult.