The timing of implantation, duration of gestation and energetic costs associated with foetal development were examined in the north-west Atlantic grey seal Halichoerus grypus. Implantation occurs between 5 and 8 May. Active gestation lasts for 259–272 days and the duration of suspended development is 78–91 days. The duration of active gestation was slightly shorter among north-east Atlantic grey seals (238–255 days), while suspended development was longer (95–112 days). The rate of foetal growth increased in August, 3 months after attachment, until mid December when it began to slow once again, c. 1 month before parturition. During August, the placenta to foetus mass ratio was about 0.3, but declined to around 0.07 near birth. Foetal energy density increased rapidly reaching asymptotic levels of 22.6 kJ g−1 (dry weight) 3–4 months after implantation. Energy density of the placenta changed only slightly throughout gestation increasing from c. 21.7 to 23 kJ g−1 (dry weight) by the autumn. Energy costs associated with foetal growth are c. 648 MJ.