Evaluation of: Lumbers ER, Kim MY, Burrell JH et al. Effects of intrafetal IGF-1 on growth of cardiac myocytes in the late gestation sheep. Am. J. Physiol. 296(3), E513-E519 (2009). A range of growth factors, including IGF-1, play a role in the prenatal growth and maturation of the heart in humans and sheep. IGF-1 was infused into the sheep fetus for 4 days from 120 days of gestation to test the hypothesis that IGF-1 increases fetal heart weight and cardiomyocyte growth. However, IGF-1 infusion had no effect on heart weight. IGF-1 infusion increased cardiomyocyte volume in male fetuses only, leading the authors to investigate the effect of sex on cardiomyocyte growth and development. At 128 days of gestation, females had a lesser proportion of mononucleated cardiomyocytes than males. The volume of these cardiomyocytes was greater in females than in males. Lumbers et al. were the first to investigate the effect of sex on cardiomyocyte growth and development during fetal life.