The structure of the porcine uterine-conceptus interface is associated with gestational day, fetal size and sex.

This study aimed to characterize histological changes of the maternal-conceptus interface in feto-placental units associated with fetal weight and sex throughout pregnancy. Pregnant Large-White X Landrace gilts(n=18) were euthanized and hysterectomized on gestational days[GDs] 30(n=3), 45(n=5), 60(n=5), and 90(n=5). Intact cross-sections of fetoplacental interface associated with the lightest[LW] and normally-grown[NW] littermates were collected on GD30(n=4 per size). On GDs 45, 60 and 90, interactions between fetal size and sex were investigated in light-weight males[LWM] and females[LWF]; normal-weight males[NWM] and females[NWF] (n=4/group/GD). Fetal weight did not affect the endometrium composition, including relative proportion of glandular epithelium, blood vessels, and connective tissue. Feto-placental units from LW embryos tended to have longer chorioallantoic fold length on GD30(P=0.06). On GD45, higher proportion of larger endometrial glands was observed in NWM, and taller trophoblastic epithelium in NW conceptuses, regardless of sex(P<0.05). NWF presented the greatest proportion of subluminal endometrial epithelial blood vessels(P<0.05). On GD60, more blood vessels were present at the folds' base in males feto-placental units, whereas taller trophoblastic epithelium were present in NWF fetuses' feto-placental units(P<0.05). Feto-placental units' morphological composition throughout gestation in NW and LW conceptuses revealed that fold length was higher as early as GD30, with no further increase up to GD90 in LW conceptuses(P>0.05). Increased proportion of glandular epithelium was observed in LW conceptuses; the highest percentage present on GD90(P<0.05). Collectively, we demonstrated that fetal weight and sex influence the morphological structure of feto-placental units from as early as GD30, suggesting potential differences in the ability for nutrient transport.