Prolific Chinese Meishan pigs have an ovulation rate (16-18 ova), fertilization rate and uterine size similar to females of U.S. pig breeds yet farrow three to four more piglets per litter. To determine the mechanism(s) for this increased litter size two studies focused on breed comparisons of the interaction between uterine type and fetal genotype during late gestation. A third study examined the effects of utilizing a selection criterion based on information obtained in the first two studies to increase litter size, piglet survivability, 21-d weights and days to 105 kilograms;Experiment 1. The surface area of Yorkshire placentae doubled in size (P <.001) between days 90 and 110 of gestation while placental and adjacent endometrial vascular density remained relatively constant. In contrast, there was no increase in placental surface area between day 70 and day 110 in the Meishan pig. Instead, the vascular density of Meishan placentae and adjacent endometrium increased markedly (P <.05). As a consequence the ratio of fetal weight/placental weight (placental efficiency; RATIO) was greater for Meishan compared to Yorkshire conceptuses;Experiment 2. Meishan x Yorkshire crossbred conceptuses were 40% smaller when recovered from Meishan or Yorkshire uteri on either day 90 or day 110 of gestation. While fetal weight increased (P <.001) between day 90 and day 110, regardless of uterine environment, placental size remained unchanged. Additionally, the vascular density of placentae and the associated endometrium were similar, regardless of uterine type and remained constant from day 90 to day 110 of gestation. RATIO was similar for conceptuses in either uterine type. While placental weight was significantly correlated with RATIO (r = -.66, P <.001), fetal weight was not;Experiment 3. No significant association was seen between RATIO and piglet weight at 21-days, days to 105 kilograms, back fat depth or loin muscle area in a herd of purebred Yorkshire and Landrace breeding stock. Thus an above average RATIO is indicative of a smaller more efficient placenta which appears to have no detrimental effects on growth rate;Combined, these experiments indicate that selecting for females with the capacity to limit placental growth should maintain an optimum litter size. Additionally, selecting piglets with a high placental efficiency would not inhibit economically important production traits.