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The aim in this experiment was to examine the effects of prepartum supplementation, fat score (FS) and gastrointestinal nematode (GIN) infection on whole-body composition of twin-bearing and rearing Merino ewes. These effects were then related to lamb birth weight, ewe colostrum yield and foetal brown adipose tissue stores as principal determinants of neonatal mortality. The experiment was conducted as a 3 x 2 x 2 factorial design from day (d) -98 to weaning at d 55 (relative to the mid-point of lambing, d 0). There were three infection groups (uninfected, 1000 Haemonchus contortus L3/week, or 6000 Trichostrongylus colubriformis L3/week), two levels of nutrition from d -97 to d -52 (designed to achieve FS targets of 2 and 4) and two prepartum supplementation groups (0 or 200 g/d cottonseed meal; CSM) from d -52 to d -12. Ewes managed to higher FS during mid-pregnancy were 8 kg heavier and 1.3 FS units greater (p<0.001) by d -54, 4 kg (p=0.049) and 0.6 FS units (p<0.001) by d -12, but did not differ from low FS ewes by d 53. Carcass fat, omental fat and lean muscle mass (all p<0.001) were greater in high FS ewes at d -12 but not at d 55. Neither colostrum yield, milk yield or lamb birth weight differed between FS groups but lambs born to low FS ewes had greater (p=0.050) brown adipose tissue reserves. Prepartum supplementation increased ewe live weight (p=0.049), carcass fat (p=0.032) and muscle (p=0.013) by parturition and lamb birth weight (p=0.012), with the benefit being greatest for low FS ewes. T. colubriformis infection reduced lamb birth weight and H. contortus reduced ewe live weight, milk yield and lamb growth to weaning. These results highlight the effectiveness of providing adequate nutrition to twin-bearing Merino ewes in late pregnancy and controlling GIN infections, especially where ewe FS has fallen below Australian industry targets. [ABSTRACT FROM AUTHOR]