Fifty-four pregnant Holstein-Friesian cattle were divided into 3 groups according to their gestation length (A=274-280 days; B=281-285; C=286-290). Changes in plasma estrone sulfate (E1S) concentrations during pregnancy, calf birth weight, calf viability, placental weight and placental expulsion were compared among the 3 groups. Differences in E1S concentrations among these groups became evident after day 271 of pregnancy and E1S levels in groups A and C were lower than in group B. In male calves, birth weights in group A were significantly lower than those in groups B (P<0.01) and C (P<0.05). Likewise, in female calves, birth weights in group A were significantly lower than those in group C (P<0.05). There were no significant differences in birth weights of both male and female calves between groups B and C. Incidences of weak calves and retained placenta were lower in group B (11.5%) than in groups A (25.0%) and C (43.8%). Cotyledonary weights in group B tended to be higher than in groups A and C, and there were significant differences between groups A and B for dams with male neonates and for total dams, and between groups B and C for total dams. Thus, gestation length below the herd average is associated with low calf birth weight, low cotyledonary weight and low plasma E1S concentrations, while gestation length above the herd average is not necessarily associated with high calf birth weight, high cotyledonary weight or high plasma E1S concentrations. The longer gestation period, however, resulted in higher incidence of neonatal weakness and retained placenta. These results indicate that the feto-placental growth slows or ceases when a gestation period extends over a normal period and as a consequence prevalence of retained placenta and weak calves increases in cows with longer gestation period.