When Meishan (M) and Yorkshire (Y) embryos were cotransferred to Y recipients that were then allowed to farrow, it was observed that the birth weights of M and Y littermates were similar averaging 1.15 ± .06 kg. In contrast, placentae matched to M piglets were markedly smaller (.70% lighter) and more vascular (.two-fold) than Y placentae. To investigate the effect of uterine environment on conceptus development to term, M and Y embryos were cotransferred to M recipients (n=3) that were slaughtered one day before expected parturition (day 113). Fetal weight, placental weight, and placental surface area were recorded. Additionally, a section of the intact maternal placental interface was excised, fixed, embedded, sectioned, and stained to allow quantitation of the placental vascular density (PVD). As observed when M and Y fetuses were cogestated by Y recipients, littermate M and Y fetuses gestated in M uteri were similar in weight (1.04 ± .03 vs. 1.03 ± .05 kg) at term. Further, M conceptuses exhibited markedly reduced (P<.03) placental weights (170 ± 19 vs. 249 ± 10 g) and surface areas (1017 ± 70 vs. 1506 ± 96 cm 2 ) compared with their Y littermates. As was the case for conceptuses gestated in Y uteri, the similarity in fetal weight between the two breeds with very different placental sizes appears to result from an increased (P<.09) M PVD compared with littermate Y fetuses (2.5 ± .3 vs. 1.4 ± .4%). These data indicate that in both the M and Y uterine environment, the reduced size of the M compared with Y placenta is compensated for by an increase in PVD. Currently it is unknown whether M placentae contain a greater amount of vasculature (total volume) or a similar amount of vasculature simply squeezed into a smaller space. To investigate breed differences in total placental vasculature we have perfused placentae of M and Y conceptuses cogestated in M uteri on d 113 of gestation with vascular casting material and will compare the volumes of the corrosion casts.