Epidemiological studies in humans linking adult disease to growth in utero indicate that prenatal life is a critical period for the appropriate development of the reproductive axis. The aim of this study was to compare ovarian development in intrauterine growth-retarded and normally grown piglets originating from the same litter. Intrauterine growth-retarded piglets (runts) were identified on the basis of statistical analysis of the birth weight distribution within each litter. At birth, ovaries were collected from runt piglets (n=14) and their respective mean weight (normal, n=14) littermates. Ovaries were weighed and fixed, and development of ovarian germ cells was quantified in haematoxylin-eosin-stained paraffin wax sections using an image analysis system. Germ cell loss, using an in situ TdT-mediated dUTP nick-end labelling (TUNEL) assay for DNA fragmentation, and follicle cell activity, using immunohistochemistry to demonstrate vimentin, were studied in ovarian sections. At birth, body weight and absolute ovarian mass were significantly lower in runt piglets compared with their respective normally grown littermates (body weight: 733+/-38.5 versus 1530+/-39.7 g; ovarian mass: 51+/-3.0 versus 108+/-9.6 mg; P<0.001 for both). In the ovary, the proportion of nests of oogonia, the number of oocytes and TUNEL-positive cells, and the localization and intensity of vimentin immunoreactivity were not different between runt and normal littermates. However, runt piglets had more primordial follicles (268+/-18.6 versus 235+/-20.1 per mm(2) of cortex; P<0.05), fewer primary follicles (11+/-2.0 versus 20+/-3.0 per mm(2) of cortex; P<0.001) and no secondary follicles compared with normal piglets. These findings indicate that intrauterine growth retardation delayed follicular development in pig ovaries at birth.