The ability of the uterus to accommodate a finite amount of placental tissue appears to be a major limitation to litter size. Meishan preimplantation conceptuses contain fewer cells, produce less estradiol-17β, elongate to a shorter length, and exhibit a reduced placental size throughout gestation than Yorkshire conceptuses. Uterine luminal embryonic estradiol-17β and growth factor content are positively associated at elongation. Based on these data, we have argued that growth factor quantity regulates the length an embryo attains at elongation, and ultimately limits placental size. Recently, we injected Meishan gilts every 6 hours with estradiol-17β on day 12 and 13 of gestation, resulting in a 40% increase in placental size at term compared with vehicle-injected Meishan gilts. This study was conducted to determine if transfer of embryos into the oviducts of asynchronous females (more or less advanced uterine environments) would alter fetal and/or placental size at term. Embryos (1 to 4 cells) were flushed from the oviducts of each donor gilt on day 2.5 of gestation and transferred in equal numbers to the oviducts of a recipient gilt on day 1.5, 2.5, or 3.5 of their estrous cycle. Gilts were slaughtered on day 112 of gestation and fetal and placental weight, placental surface area, and implantation site lengths were determined. Although litter sizes were similar (8.4 ± 1.1), conceptuses transferred to day 3.5 recipients had heavier fetuses (1.57 ± .09 vs. 1.23 ± .04 kg, P<.001), larger placental surface area (1812 ± 106 vs. 1458 ± 43 cm 2 , P<.01) and occupied longer implantation site length (34 ± 3 vs. 25 ± 1 cm, P<.001) than those transferred to recipients on day 1.5 or 2.5. These data demonstrate that oviductal transfer of embryos to a reproductive tract as little as 24 hours more advanced can result in dramatic alterations in placental growth and function during gestation.