High piglet birth weights and litter uniformity are important for piglet survival and piglet performance. Within-litter variation in piglet birth weight is the consequence of within-litter variation in early embryo development, which in turn reflects variation in follicle and oocyte development. Insulin-stimulating diets before mating can influence litter development and uniformity, probably through beneficial effects of insulin on IGF-1 and follicle development. The first aim of this thesis, therefore, was to study effects of insulin-stimulating diets during the weaning-to-estrus interval (WEI) on plasma insulin and IGF-1 levels and follicle development, and consequences for embryo, fetal and placental development and uniformity at different stages of pregnancy in sows. Results of this thesis show that plasma insulin levels during WEI can be effectively enhanced by dietary sugars as dextrose and sucrose (high peaks directly after feeding) and starch (enhanced insulin levels at ~4h after feeding) in a dose-dependent manner. Follicle development and subsequent litter uniformity of embryos (at day 10 of pregnancy) or fetuses and placentas (at day 42 of pregnancy), however, were not affected by insulin-stimulating diets during WEI, nor related to plasma insulin and IGF-1 levels during WEI. Besides effects of pre-mating diets, plasma insulin and IGF-1 levels and follicle development are also influenced by the pre-mating metabolic state of the sow: in sows with severe body condition loss during lactation, plasma insulin and IGF-1 levels and follicle development at weaning are suppressed, and restoration of plasma insulin and IGF-1 levels and follicle development occurs in sows with a prolonged lactation or a prolonged weaning-to-pregnancy interval (WPI). The second aim of this thesis, therefore, was to study effects of these pre-mating conditions related to sow metabolic state on subsequent piglet birth weight and litter uniformity. In this thesis it is shown for the first time that pre-mating conditions related to sow metabolic state affect subsequent litter uniformity. Litter uniformity at birth was compromised by severe sow body condition loss during previous lactation and improved in sows with a prolonged WPI. Furthermore, it was shown that in (organic) sows with prolonged lactations (6 weeks) and large litters (17.4±0.3 piglets), insulin-stimulating diets before mating did not result in improved piglet birth weights or litter uniformity. This may be related to a restored follicle development at weaning in these sows. In these large organic litters, piglet birth weight and litter uniformity were strongly related to piglet survival during lactation. To conclude, results of this thesis confirm that litter uniformity at birth is already (partly) determined during the pre-mating period, likely related to (insufficient) restoration of follicle development. In contrast to previous studies, insulin-stimulating diets during WEI did not improve litter uniformity of embryos, fetuses or placentas and/or piglets in sows. The role of plasma IGF-1 levels and follicle development at weaning (both related to sow parity and sow body condition loss), and effects of insulin-stimulating diets during lactation, need further study. Finally, although effects of pre-mating nutritional and metabolic conditions on subsequent piglet birth weight and litter uniformity seem only marginal, these marginal effects can have substantial effects on pre-weaning piglet survival.