Low birth weight has detrimental consequences for postnatal well being and survival. Therefore greater understanding of the factors that govern birth weight are important. There are many extrinsic and intrinsic factors that ensure the adequate growth of an offspring; these include maternal nutrition, maternal health, placental size, surface area and placental transport capacity. This study has used the runt pig fetus as a naturally occurring model of low weight; to better understand intrinsic factors that may contribute to low birth weight. The lightest fetuses and a normal size fetus from Large while X Landrace pig were studied at day 45,65 and 100 of gestation (term being 112-115 days). Placental sodium transport was examined. In relation to sodium transport it was observed that the short circuit current across the pig placenta was predominantly carried by sodium in a fetal maternal direction and involved both the epithelial sodium channel (ENaC) and the sodium pump This thesis is the first to report the presence of ENaC in the pig placenta. However fetal growth depends not just on the ability of the placental to grow to an appropriate size and to transfer nutrients to the fetus, but fetal growth must be appropriate to nutrient supply if the fetus is to survive to term. Therefore by looking at fetal concentrations of hormones that regulate fetal grow and differentiation we may better understand what causes low birth weight in pigs. In this thesis it was observed that no relationship existed between fetal size and fetal plasma insulin, T3 and T4 concentrations at day 100 of gestation. However low weight pig fetuses were found to have lower plasma cortisol concentration on day 45 of gestation and higher plasma cortisol concentrations of day 100 of gestation compared to their normal size littermates. Elevated cortisol concentrations on day 100 of gestation were not related to changes in fetal plasma ACTH concentrations.