Placentae from control and diabetic subjects were analysed using stereological techniques in order to assess the effects of mode of delivery (vaginal versus caesarean) and sex of neonate on parenchymal morphology. Effects were assessed using indices of peripheral villous and fetal capillary growth, villous maturity, extent of maternal intervillous space and thickness of intervascular tissue layers. Placentae were from pregnancies (37-42 wk) which were either uncomplicated (control group) or complicated by diabetes mellitus (diabetic group, White class D) which was reasonably well controlled in terms of glucose and glycated haemoglobin levels. Neonatal and placental weights were recorded and placentae sampled in a systematic random fashion. Fields of view on formalin-fixed, paraffin-embedded sections were analysed to obtain estimates of volumes, surface areas, lengths and diffusion (harmonic mean) distances. Comparisons were drawn using 3-way analyses of variance with group, mode of delivery and sex as the principal effects. The mean length of gestation was 2 wk longer in controls (39 versus 37 wk). Despite this, mean birth weight was similar (3.5 kg) in control and diabetic groups. Moreover, diabetic placentae were 17% heavier and showed shorter fetal plasma distances (30%) and larger fetal capillaries (volume 45%, surface 39% and length 30% greater). Mode of delivery had significant main and interaction effects on stromal diffusion distance (25% greater in vaginal deliveries) and an interaction effect on fetal capillary volume. Sex had significant main effects on the maternal plasma distance (21% greater in males) and capillary volume (30% bigger in males) and an interaction effect on placental weight and mean capillary diameter.(ABSTRACT TRUNCATED AT 250 WORDS)