Evolutionary theories based on adaptive modification of maternal investment are generally suggested to explain variation in offspring sex ratios, but general concordance with empirical data is rare. Recently, factors beyond the control of the mother, such as density and climate, were reported to affect offspring sex ratios, and the extrinsic modification (EM) hypothesis has been advanced to explain such variation. The EM hypothesis states that offspring sex ratios in sexually dimorphic vertebrates may vary with extrinsically induced changes in maternal condition during pregnancy. We investigated sex ratio variation and inter-cohort calf live weight variation in 5,509 reindeer Rangifer tarandus calves during an eight-year period. Using separate models, we also tested whether offspring sex ratio covaried with density, and, respectively, summer weather before conception and winter weather (measured by the North Atlantic Oscillation (NAO)) when calves were in utero. Offspring sex ratios in mid-July differed from parity only in one year. Calf live weight varied among cohorts and males were heavier than females. The proportion of male calves in the herd decreased significantly with an increasing NAO index when calves were in utero. Density in the year of birth only affected calf sex ratios by increasing the negative effects of the NAO index. The proportion of males in the herd also declined with increasing summer temperature, summer precipitation, and density in the year of conception, suggesting potential for pre-conception environmental effect. Because the reported environmental variables affected negatively the proportion of male calves in the herd, we suggest that their effects may be associated with nutritional stress caused to the mother before conception, which have not been shown previously, and during pregnancy. Evidence of extrinsic sex ratio adjustment was conclusive and our findings therefore support the EM hypothesis of sex ratio variation.