Many litters contain at least one piglet that is significantly smaller than its littermates. Such piglets have increased perinatal mortality and reduced carcass quality. Tissue accretion and differentiation are regulated by a number of hormones including cortisol, which has the ability to stimulate or inhibit tissue growth and differentiation. We therefore investigated whether differences in porcine fetal size on days 45, 65 and 100 of gestation are associated with altered fetal cortisol concentrations and if these changes are mediated by changes in fetal plasma adrenocorticotropic hormone (ACTH) levels.

Large White X Landrace multiparous sows were exsanguinated under deep anaesthesia (8 % v/v halothane) in accordance with UK legislation on days 45 (N = 7), 65 (N = 5-9) and 100 (N = 6-20) of gestation. An average-sized (av) and the smallest fetus (sm) from each litter were identified by weight and fetal cardiac blood collected immediately prior to killing (barbiturate overdose). Plasma concentrations of cortisol and ACTH (days 65 and 100 only) were determined by radioimmunoassay and the effects of fetal size and day of gestation were analysed by ANOVA.

Plasma cortisol (mean ± S.E.M.) was at day 45 sm: 16.84 ± 3.69, av: 34.42 ± 3.41 ng ml^-1, day 65 sm: 5.69 ± 0.99, av: 4.75 ± 0.53 ng ml^-1 and day 100 sm: 10.68 ± 1.52, av: 7.55 ± 0.72 ng ml^-1. The smallest fetuses had significantly lower cortisol levels at day 45 and higher cortisol levels at day 100 (P < 0.05) when compared with average-sized fetuses. Cortisol levels were significantly different between the gestational days (P < 0.05). Plasma ACTH (mean ± S.E.M.) at day 65 was sm: 245.94 ± 17.55, av: 281.72 ± 33.91 pg ml^-1, and day 100 was sm: 419.44 ± 73.66, av: 320 ± 34.28 pg ml^-1. There was no significant difference in plasma ACTH between the smallest and average-sized fetuses, or between days 65 and 100 of gestation.

It is concluded that elevated cortisol levels in the smallest fetuses of each litter at day 100 of gestation are not caused by elevated plasma ACTH, and therefore may reflect reduced placental 11-βhydroxysteroid dehydrogenase activity or increased adrenal activity in these small fetuses.

This work was supported by SERAD Flexible funds. We would like to acknowledge Dr A.F. Parlow (National Hormone and Peptide Program, California, USA) for the gift of human ACTH and ACTH antiserum, and Diagnostic Scotland for the gift of donkey anti-rabbit IgG and rabbit serum.