Lambs born in the autumn or winter have substantially lower birth weights than those born in the spring, but the physiological basis of this difference is unknown. This study examined the effects of season on foetal growth and placental development in ewes managed under controlled grazing to minimise the confounding effects of maternal liveweight change. Mature Romney ewes pregnant to matings in December (n= 13) or March (n #x2010;13), with similar liveweights at mating, were managed to achieve similar liveweights at Day 140 of gestation. At that time, measures of foetal growth and placental development, adjusted for litter size, were (December#x2010; versus March#x2010;mated): foetal weight (4.15 #xB1; 0.16 versus 5.07 #xB1; 0.16 kg,P 0.001); total foetal weight per ewe (6.42 #xB1;0.18 versus 7.14 #xB1; 0.22 kg,P 0.01); caruncle number (114.5 #xB1; 4.1 versus 121.0 #xB1; 4.2,P 0.10); placentome number (89.4 #xB1; 4.2 versus 106.9 #xB1; 4.3,P 0.01); number of placentomes per number of caruncles (0.79 + 0.03 versus 0.88 #xB1; 0.03,P 0.05); and total placentome weight (564.7 + 34.0 versus 679.0 #xB1; 34.9 g,P 0.05). Maternal circulating concentrations of prolactin, IGF#x2010;1, and growth hormone are reported. This study has demonstrated that the marked seasonal differences in foetal growth are associated with seasonal differences in placental size, the formation of placentomes being significantly reduced in December#x2010;mated ewes.
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