The influences of nutritional protein during the first and second trimesters of pregnancy on placental hormones and fetal growth were determined in composite beef heifers. At artificial insemination, heifers were stratified by weight within each composite genotype into 4 treatment groups: High High (HH = 1.4 kg crude protein (CP)/day for first and second trimesters of gestation; n = 16), High Low (HL = 1.4 kg CP/day for first trimester and 0.4 kg CP/day for second trimester; n = 19), Low High (LH = 0.4 kg CP/day for first trimester and 1.4 kg CP/day for second trimester; n = 17) or Low Low (LL = 0.4 kg CP/day for first and second trimesters; n = 19). Maternal plasma bovine pregnancy associated glycoprotein (bPAG) and progesterone (P4) were determined at gestation day (gd) 28, 82, 179 and 271 (mean gestation length 286 days) in addition to P4 at term. Estrone sulphate (ES) and bovine placental lactogen (bPL) concentrations were measured at gd 124, 179, 236 and 271 and at term in addition to ES at gd 82. Low dietary protein increased placental function as indicated by increased bPAG (P <0.001) and ES (P = 0.02) concentrations in first trimester and increased bPL concentrations (P = 0.01) in the second trimester of gestation. In the third trimester, when dietary treatment had ceased, placental function was no longer associated with previous dietary treatments. Dam genotype affected placental function as measured by bPL (P <0.001) and ES concentrations (P = 0.02). Calf gender, heifer age and maternal insulin-like growth factor (IGF)-I, -II and leptin did not affect hormonal indicators or circulating markers of placental function. Enhanced placental function during the third trimester, as measured by ES, was associated with increased calf birth weight (P = 0.003).