The energy requirements of the fetus through pregnancy have been previously investigated in several slaughter experiments, and many studies have attempted to estimate the effect of pregnancy on milk production. Data from 154 pasture-based dairy cows (77 twin pairs) were subjected to a retrospective analysis to determine the effect of pregnancy on milk production and body weight. A decline in milk yield from 126 d of pregnancy was observed in twins that were pregnant, but the decline was small and insignificant until 147 d of gestation (at 33 wk of lactation), after which pregnant cows produced less milk (0.8 kg/cow per day). Protein and fat concentration increased in pregnant cows from 77 and 133 d of gestation, respectively. The yield of milk fat and protein was not affected by pregnancy until 168 d of gestation, after which pregnant cows produced less milk fat (0.06 kg/cow per day) and milk protein (0.04 kg/cow per day) compared with their nonpregnant twins. Body weight was higher in pregnant twin cows after 90 d of gestation and continued to increase until the end of lactation (182 d of pregnancy). Fitted splines showed higher milk protein concentrations in pregnant cows throughout lactation, and milk protein yield was higher (20 g/cow per day) during the breeding season (between 12 to 17 wk of lactation) in pregnant twins. Higher milk yield and energy secreted in milk (net energy for lactation) were also found in the pregnant twins during the breeding season. Although pregnancy-related reductions in yields of milk, milk fat, and milk protein occurred after midgestation, overall effects of pregnancy on milk production in this pasture-based, seasonal calving system were small.