Meishan pigs farrow 3--5 more pigs per litter than our domestic commercial breeds, while exhibiting a similar ovulation rate and uterine size. The greater fecundity of the Meishan is primarily due to its reduced placental size, coupled with an increased placental efficiency (ratio of fetal weight to placental weight). The fact that Meishan embryos grow more slowly very early in gestation and elongate to a shorter length around day 14 of gestation, when compared to Yorkshire embryos, potentially results in the smaller placental size of the Meishan later in gestation. In order to determine if placental size and efficiency are heritable, we characterized the variation in placental size and efficiency within our Yorkshire herd. We then selected and bred boars and gilts that either developed on small, efficient placentae or large, inefficient placentae. Gilts which developed on a small, efficient placenta gave birth to significantly larger fitters than did their contemporaries which developed on large, inefficient placentae (12.6 vs 9.5 pigs per litter). These data suggested that not only was placental size and efficiency a key component of determining litter size, but that commercially relevant pig breeds had the potential to achieve large litter sizes based on selection for placental size and efficiency. We then conducted two experiments to investigate if altering embryonic growth prior to, or during, elongation could alter placental size at term. First, we administered estradiol-170 to pregnant gilts during elongation, which is known to increase stimulatory growth factor content in the uterine lumen, and stimulated a 40% increase placental size at term. Second, we transferred day 2.5 embryos to gilts that were day 3.5 of their estrous cycle (likely altering the timing and/or level of exposure of embryos to endometrial growth factors) and were again able to markedly increase placental size at term. Based on our new insight that early embryo growth directly impacts litter size in the pig, we began a characterization of gene products that are turned on or off in association with preimplantation embryonic growth and elongation to better understand the regulation of conceptus growth, which ultimately limits litter size.