This study examined the physical and physiological differences from birth until 12 h of age within twin- and triplet-born litters. In 2005 and 2006, the parturition of 75 twin- and 62 triplet-bearing Romney ewes were observed. After parturition lamb blood samples were taken within 5 min of birth and rectal temperature was measured within 5 min of birth and at 1, 3, 6 and 12 h post-birth. Lamb birth weight, crown–rump length and thoracic-girth circumference were measured at 3 h of age. Lamb birth weight, plasma glucose, fructose, lactate, thyroxine and tri-iodothyronine did not differ (P > 0.05) between the heaviest-twin-born, lightest-twin-born and heaviest-triplet-born lamb. The lightest-triplet-born lambs, however, had lighter (P < 0.001) birthweights, greater (P < 0.001) surface-area-to-birth weight ratio, lower (P < 0.05) rectal temperatures, greater (P < 0.1) lactate concentrations and lower (P < 0.1) plasma thyroid hormone concentrations. These characteristics are known to have a negative impact on the ability of the lamb to maintain its body temperature after birth and may therefore provide some evidence as to why triplet-born lambs display a greater mortality rate than twin-born lambs, and why the lightest-triplet-born lambs have the greatest mortality rate within a litter.