Submitted to: Animal Reproduction Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2011
Publication Date: 5/1/2011
Citation: Vallet, J.L., Freking, B.A., Miles, J.R. 2011. Effect of empty uterine space on birth intervals and fetal and placental development in pigs. Animal Reproduction Sciences. 125:158-164.
Interpretive Summary: Stillbirth in pigs occurs at a rate of 2 to 10% of piglets born and represents a significant inefficiency in swine production. Stillbirth is associated with prolonged birth of a piglet, so factors influencing how long it takes for the sow to deliver each piglet during the birth process are of interest to develop strategies to reduce delivery times. The time required to deliver each piglet is reduced as the number of piglets in the litter increases, but the mechanism responsible is not known. We hypothesized that a small number of piglets in a litter results in significant empty space in the uterus, and it is known that these empty spaces are more constricted than adjacent spaces containing piglets. We further hypothesized that this constriction could impede the delivery of piglets located behind the constriction and therefore contribute to the longer delivery time of piglets in small litters. To test this hypothesis, we created an empty uterine space in the uterus between the entire litter and the exit of the uterus by removing one or two fetuses during surgery on day 35 of pregnancy. As a control, in a second set of pregnant pigs, we removed one or two fetuses in the uterus such that the empty space was not between any piglets in the litter and the uterine exit. A subset of pigs was slaughtered at 105 days of pregnancy to confirm that the empty regions were successfully created, and at the same time the effect of removal of fetuses on day 35 of pregnancy on the growth of adjacent fetuses and their placentas was examined. The rest of the treated pregnant pigs were allowed to deliver their litters and the delivery of each piglet was monitored by 24-h video recording to determine effects of the empty space before or after the litter in the uterus on the average time to deliver each piglet. Results indicated that removal of fetuses on day 35 of pregnancy allowed adjacent placentas to partially grow into the empty uterine space, but was successful in creating a constricted empty uterine space. Despite this, there was no difference between the average time required to deliver piglets when the empty space was in front of the litter versus when it was behind the litter relative to the uterine exit. These results indicate that empty uterine space does not contribute to the prolonged piglet delivery times that occur in small litters. Identification of the mechanism by which small litters experience prolonged piglet delivery times will lead to strategies to reduce piglet delivery times, thus reducing the incidence of stillbirth in pigs.
Technical Abstract: A substantial loss of embryos occurs between d 30 and 40 of pregnancy in the pig under crowded intrauterine conditions, but it is not known whether this loss affects the growth of the remaining adjacent fetuses. Birth intervals are known to decrease with increasing litter size, but the factors causing this association are unclear. One possibility is that the empty uterine space and the resulting constricted uterine regions that occur in pigs with small litters may impair piglet births. To answer these questions, pregnant gilts were laparotomized on d 35 of pregnancy and one or two fetuses were manually crushed through the uterine wall on the ovarian end or cervical end of each uterine horn. This was done to create an empty uterine space behind or in front of the litter of piglets, respectively, in relation to the route of delivery from the uterus. A subset of gilts was slaughtered at 105 d of gestation to confirm that the empty uterine spaces were successfully created and to determine the effects of those spaces on placental and fetal weights of remaining adjacent conceptuses. At slaughter, the lengths of all externally visible empty constricted regions of the uterus were measured. The uterine horns were opened and the lengths of each placenta were measured from the umbilicus toward the ovary and toward the cervix to assess whether placentas developed symmetrically, and then each fetus and placenta was weighed. Fetal crushing successfully created constricted empty uterine regions on the ovarian and cervical ends of the uterine horns. Placental lengths were greater toward the ovary compared to toward the cervix for conceptuses adjacent to fetuses crushed on the ovarian end of the horn. Placental lengths were greater toward the cervix compared to toward the ovary for conceptuses adjacent to fetuses crushed on the cervical end. Both placental and fetal weights were greater (6% and 10%, respectively, P < 0.05) for conceptuses adjacent to crushed fetuses compared to nonadjacent conceptuses. The remainder of the gilts was farrowed to determine the effect of treatments on birth intervals of piglets, which were monitored using 24-h video surveillance. Despite the successful creation of empty uterine space in cervically-treated gilts, birth intervals did not differ between cervically- and ovarian-treated gilts. These results indicate that conceptus loss on d 35 of gestation benefits the growth of adjacent placenta and fetuses, but the benefit is small. In addition, the presence of empty uterine space when litter size is reduced does not significantly prolong birth intervals.