Intrauterine position and adjacent fetal sex affects fetal and placental growth throughout gestation, but not embryonic viability, in pigs selected for component traits of litter size

Abstract Intrauterine position and sex of adjacent fetuses in litter bearing species have been implicated in physiological and behavioral differences of offspring. The effects of uterine position and sex status of flanking fetuses with crowded uterine conditions on fetal and placental growth rate was tested. Gilts were unilaterally hysterectomized-ovariectomized at 160 d of age and mated at approximately 280 d of age, with fetal harvest at 45, 65, 85, or 105 d of gestation. Uterine position relative to the cervix, fetal status (alive, dead, sex), fetal weight, and placental weight were recorded at harvest. Each fetus was coded as adjacent to 0, 1, or 2 opposite sex fetuses and analyzed using an ANOVA fitting contemporary group, line, and flanking fetal sex code as fixed effects with sire as a random effect. The fraction of live fetuses in each classification (0, 1, 2) was 26.4%, 50.1%, and 23.4%, respectively, indicating no effect on fetal survival. Fetal weight was affected by flanking sex status between 65 d (P