All Fetuses Are Not Equal: Spatial and Developmental Determinants of Within-Litter Variation in the Pig
Yvette E. Wolpo, Claire StenhouseAbstract
Within-litter variation in fetal growth is a feature of pregnancy in polytocous species such as the pig, yet it is often masked by analytical approaches that treat the litter as homogeneous. Selection for increased ovulation rate and litter size has amplified this variation, increasing numbers of low-birthweight and intrauterine growth-restricted piglets with reduced survival and performance. Evidence indicates that divergence in fetal growth trajectories is established early in gestation and is shaped by constraints at the level of the uterus and placenta. Uterine capacity, the maximum number of fetuses a uterus can support to term, is a constraint on reproductive success. As the number of conceptuses (embryo/fetus and associated extraembryonic membranes) approaches this limit, competition for uterine space and resources may restrict placental size, exchange surface area, and nutrient transfer capacity. Importantly, uterine capacity differs from uterine size, as the dimensions of the uterus do not necessarily determine the number of fetuses it can support. These constraints interact with spatial organization within the uterus, including intrauterine position and conceptus spacing, resulting in regional differences in resource availability. Biological factors intrinsic to the conceptus further contribute to within-litter heterogeneity. Fetal sex influences placental and endometrial function, while local interactions among neighboring fetuses, including variation in adjacent fetal sex and litter sex ratio, may modify the intrauterine environment and developmental outcomes. This review integrates current understanding of the spatial and developmental determinants of within-litter placental and fetal heterogeneity in the pig and highlights necessary experimental approaches that account for sources of variation.
Lay Summary
In pigs, not all piglets grow the same before birth, even when they develop in the same pregnancy. However, these differences are often overlooked. This review describes some of the reasons why piglets within a litter can grow differently. As litter size increases, piglets must share limited space and nutrients in the uterus, creating competition during development. Differences can begin very early in pregnancy and may depend on where each fetus is located in the uterus. The sex of the fetus and neighboring fetuses may also influence growth and development. Together, these factors mean that each fetus develops under different conditions, which can affect birth weight, survival, and later performance. Understanding why these differences occur may help identify ways to produce more uniform litters and improve piglet survival and productivity.