The body composition and metabolic status of twin- and triplet-bearing ewes and their fetuses in late pregnancy

This study investigated the body composition and metabolic status of twin- and triplet-bearing ewes and their fetuses in late pregnancy. The aim was to identify factors that could explain the lower survival rates of triplet lambs compared with twin lambs. Ten twin- and 10 triplet-bearing ewes and their fetuses were euthanased on either day 139 or 140 of pregnancy and fetal and placental measurements recorded. The weights of the gravid uterus (P < 0.01) and mammary gland (P < 0.05) were significantly heavier (although not proportionally), in triplet-bearing than twin-bearing ewes (21.10 vs. 16.85 and 1.08 vs. 0.80 kg respectively). There were no other differences in the weight of ewe organs or in metabolite concentrations. The number of cotyledons per triplet fetus was significantly (P < 0.001) less compared to twins although this was countered by a tendency for greater (P = 0.05) individual cotyledon weight, resulting in no differences in either total fetal membrane plus cotyledon or total cotyledon weights.The absolute weight of many organs were significantly (P < 0.05) heavier in twin fetuses than in triplet fetuses. However, after correction for fetal body weight the only differences observed were significantly (P < 0.05) lighter, by approximately 10%, semitendinosus muscle, liver and heart weights in triplet compared to twin fetuses (6.2 vs. 5.6, 88.9 vs. 80.4, 33 vs. 30 g respectively). The fetal fructose concentrations were significantly (P < 0.05) lower in triplet than in twin fetuses.The mean weight of the lightest triplet fetus within each triplet set was 31% lighter than that of the heaviest triplet (P < 0.05), resulting in significantly (P < 0.05) lighter absolute weights of most fetal measurements taken. However, after correction for fetal body weight these differences disappeared. Within twin sets, the mean weight of the lightest fetus was only 16% lighter (P < 0.05) than its heavier counterpart but there were no significant differences in absolute organ weights. When corrected for fetal body weight, the lightest twin fetus had significantly (P = 0.05) greater kidney weights and tended to have heavier heart (P = 0.07) and lungs (P = 0.07). It is unknown why the relative organ weight relationships between the heaviest and lightest differ between twin and triplet fetuses.In conclusion this study has identified three factors which may contribute to the lower survival rates observed in triplet-compared to twin-born lambs; low fructose concentrations, lighter fetal weights, especially in one fetus within each triplet set, and maternal mammary gland increases that are not proportional to the increase in numbers of fetuses. While these findings are not novel, there were no other factors identified that could account the lower survival rates reported in triplet-born lambs compared to twins. The other only other significant differences identified were the proportionally lighter livers and hearts in triplets although the possible effects of these on survival is unknown.