Local changes in uterine dimensions in the pregnant pig

Twenty gilts were used in a study to determine local utilization of space by fetuses in the uterus under crowded or roomy conditions. Between Days 2 and 4 after onset of estrus and mating, the uterus was ligated to leave one and one-half times as much space for embryos in one side of the uterus (roomy) as in the other (crowded). At a second surgery on Day 28, serosal sutures were placed to mark the placental boundaries and every third embryo in the crowded side was crushed. Gilts were slaughtered on Day 100 and measurements taken on the uterus and fetuses. Survival from Day 28 to Day 100 of fetuses that were not crushed was 57% in the roomy side of the uterus versus 40% in the crowded side (P = 0.42). No differences were found between crowded and roomy sides for fetal crown rump length, weight or placental surface area. The length of segment occupied on Day 28 by fetuses surviving to Day 100 in both crowded and roomy sides was greater than the mean length occupied by all embryos present on Day 28. The length of segment occupied by surviving fetuses increased by 9.4 ± 1.4 cm from Day 28 to Day 100, and the length of segment on Day 28 occupied by a fetus that was crushed decreased by 2.0 ± 0.8 cm. The internal vascular area of the uterus associated with a surviving fetus on Day 100 did not extend beyond the suture placed in the serosa on Day 28, indicating that the surviving fetuses did not make use of space vacated by nonsurviving littermates.