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Modifications of Pre and Postnatal Development Due to Placental Efficiency and Maternal Exercise during Gestation in Swine Tiffany Ann Wilmoth In pigs, litter size and developmental competence are crucial for successful production of the meat product. However, the ability of the placenta to produce a developmentally competent pig is vital in the production of meat animals. Placental efficiency (fetal weight divided by placental weight) has been studied as a measure of placental function and litter size. Selection for increased placental efficiency has been shown to increase placental efficiency as well as litter size. Furthermore, formative muscle development occurs entirely during gestation, so that the number of muscle fibers the pig is born with is reflective of the number of fibers of the pork product after harvest of the animal. Following parturition, hyperplasia no longer occurs and muscle growth is entirely hypertrophic. The ratio of secondary to primary muscle fibers is an important marker for pork quality as well. The number of fibers and the size to which the muscle fibers grow, particularly the secondary muscle fibers can affect pork quality. Additionally, the growth of the secondary muscle fibers can be affected by environmental conditions (e.g., housing, stress). As shifts in industry trends take place, the effects these changes will have on the development of the fetuses of pregnant sows and gilts need to be studied. The objectives of the work here were: 1) to determine the effect of placental efficiency on in utero muscle development, 2) to determine the influence of placental efficiency on the postnatal growth performance and muscle development of pigs at market weight, and 3) to investigate the effects of exercise of second parity sows during gestation on fetal development and metabolism. For the first objective, pregnant females were ovariohysterectomized at day 95, fetal blood and muscle samples were collected and used to determine fetal metabolite concentrations and muscle fiber number and size, respectively. On day 95 of gestation, primary and secondary muscle fiber size of both loin and ham muscle samples were increased in fetuses of low placental efficiency compared to high placental efficiency fetuses. For the second objective, pregnant females farrowed naturally and pigs were caught, umbilical cords tagged and ears notched so placental efficiency could be determined following expulsion of placentae. Pigs were then fed to market weight (120 kg) and slaughtered. Muscle samples were collected following a 24 hour chill and used to determine muscle fiber number and size. Placental efficiency at birth decreased the secondary to primary ratio of muscle fibers in loin muscle samples in low placental efficiency pigs at market weight. Finally, in accordance with the third objective, second parity sows were exercised from day 44 to day 94 of gestation. On day 94, sows were ovariohysterectomized and euthanized. Fetal blood samples and muscle samples were collected to determine fetal metabolite concentrations and fetal muscle fiber number and size, respectively. Exercise during gestation decreased fetal body and muscle weights as well as increasing muscle fiber size and decreasing muscle fiber number in ham muscle samples. In conclusion, placental efficiency, and therefore placental function, plays an important role not only in the in utero development of muscle, but postnatal development, as well. Exercise during gestation negatively altered fetal development and potentially fetal metabolism, which could result in poor growth performance later in life.