Submitted to: Midwestern Section of the American Society of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: November 18, 2005
Publication Date: August 20, 2006
Citation: Vallet, J., Freking, B., Kayser, J.P., Christenson, R., Leymaster, K. 2006. What we have learned about prenatal physiology in the pig from uterine crowding experiments [abstract]. Journal of Animal Science. 84 (Supplement 2):71-72. (Abstract #95) Technical Abstract: The factor most limiting to litter size is becoming uterine capacity, the number of fetuses that the uterus can maintain until farrowing. Intrauterine crowding does not appreciably limit litter size until after day 30 of gestation, with most fetal loss occurring between day 30 and 40 of gestation, although further losses occur in later gestation. We have used unilateral hysterectomy-ovariectomy (UHO) to study the effects of intrauterine crowding on the uterus, fetus and placenta. Studies indicated few effects on uterine function, measured as endometrial protein secretion. In UHO gilts, fetal hematocrits decreased with fetal weight, suggesting that small fetuses were anemic. This observation led to the discovery of a polymorphism in the porcine erythropoietin receptor gene that is associated with litter size. Fetal brain growth is relatively resistant to intrauterine crowding, particularly during late gestation. In contrast, growth of the fetal heart is more resistant to crowding during early pregnancy. Further work on the mechanisms that shunt nutrients to various organs could provide improvements in litter size. Finally, several previous studies indicate that placental efficiency (fetal weight:placental weight ratio) increased due to intrauterine crowding. Other studies suggest that placental folding may influence the efficiency of the placenta. The depth of placental folding increased in placentas associated with the smallest fetus in a litter. This occurred at the expense of the fetal stroma layer surrounding the placental interface. During late gestation in some small fetuses, the placenta may have no further room to increase folding, potentially resulting in death. This could explain fetal losses due to intrauterine crowding that occur during late gestation. In summary, although the full effects of intrauterine crowding are still unclear, clues are beginning to unfold that could result in improvements in litter size in swine.