| Against a background of changing patterns of prenatal loss in the pig, the implications of increased numbers of embryos in utero for placental and fetal development were examined. In the first experiment, laparotomy at day 30 of gestation in gilts, and necropsy of neonatal piglets was used to investigate the relationships between placental size, levels of uterine crowding and fetal development. Although only moderate levels of uterine crowding were observed, a "brain sparing" effect in neonatal piglets was seen in the absence of effects on term placental weight, or birth weight. A second study confirmed high ovulation rates in commercial dam-line sows and established effects of parity on ovulation rate, number of conceptuses in utero, placental weight and fetal brain:liver weight ratio (a measure of intrauterine growth retardation, IUGR), as well as an interaction between parity and gestational day on prenatal loss. Further studies utilised surgical oviduct ligation (LIG) to successfully reduce the number of embryos in utero. In the third study, purebred sows of parities 4 to 6 underwent either embryo count surgeries at day 30 of gestation or LIG before breeding. In the absence of high ovulation rates in these purebred sows, the threshold for effects of intrauterine crowding on fetal development at day 30 or day 90 were not reached. In the final study, third parity, cross-bred control sows or LIG sows were slaughtered at either day 30 or 90 of gestation and the effects of numbers of conceptuses on prenatal organ and muscle fibre development were determined. The greater number of embryos in control sows exerted negative effects on placental and fetal weights, fetal brain sparing, associated with decreased muscle weights and cross-sectional areas, and a lower total number of secondary muscle fibres. The distribution of myosin heavy chain isoforms did not differ between groups. Collectively, these studies indicate that the potential for uterine crowding to impact fetal development depends on a complex interaction of a number of factors including parity, genotype and the health status of the sow. |
