We examined reproductive seasonality in 2 populations of the harbor porpoise (Phocoena phocoena) in the eastern and western North Atlantic Ocean and compared our observations with historical data from the Baltic and North seas. Dates of conception for individuals were back-calculated from fetal size and an estimated fetal growth rate of 83 mm/month. Mean conception date was 6 July ± 9.5 days (SD) in the Bay of Fundy and Gulf of Maine and 25 July ± 20.3 days in the Kattegat and Skagerrak seas. Timing of conception did not differ between the Kattegat and Skagerrak and the North seas but was significantly earlier there than in the Baltic Sea (18 August ± 11.8 days). Based on mean body size at birth (a mass of 6,025 ± 440 g and a length of 762 ± 54 mm), gestation was estimated to last for approximately 10.4 months. We hypothesize that differences in mating seasons reflect seasonal patterns in distribution and behavior of prey, so calves are born when maternal prey is abundant and is of high quality.