In the European Union, the group-housed pregnant sows have to have a minimal legal available area of 2.25 m2/sow. However, it has been observed that an increased space allowance reduces agonistic behaviour and consecutive wounds and thus induces better welfare conditions. But, what about the environmental impacts of this greater available area? Therefore, the aim of this study was to quantify pollutant gases emissions (nitrous oxide, N2O, methane, CH4, carbon dioxide, CO2 and ammonia, NH3), according to the space allowance in the raising of gestating sows group-housed on a straw-based deep litter. Four successive batches of 10 gestating sows were each divided into two homogeneous groups and randomly allocated to a treatment: 2.5 v. 3.0 m2/sow. The groups were separately kept in two identical rooms. A restricted conventional cereals based diet was provided once a day in individual feeding stalls available only during the feeding time. Rooms were automatically ventilated. The gas emissions were measured by infra red photoacoustic detection during six consecutive days at the 6th, 9th and 12th weeks of gestation. Sows performance (body weight gain, backfat thickness, number and weight of piglets) was not significantly different according to the space allowance. In the room with 3.0 m2/sow and compared with the room with 2.5 m2/sow, gaseous emissions were significantly greater for NH3 (6.29 v. 5.37 g NH3-N/day per sow; P < 0.01) and significantly lower for N2O (1.78 v. 2.48 g N2O-N/day per sow; P < 0.01), CH4 (10.15 v. 15.21 g/day per sow; P < 0.001), CO2 equivalents (1.11 v. 1.55 kg/day per sow; P < 0.001), CO2 (2.12 v. 2.41 kg/day per sow; P < 0.001) and H2O (3.10 v. 3.68 kg/day per sow; P < 0.001). In conclusion, an increase of the available area for group-housed gestating sow kept on straw-based deep litter seems to be ambiguous on an environmental impacts point of view. Compared with a conventional and legal available area, it favoured NH3 emissions, probably due to an increased emitting surface. However, about greenhouse gases, it decreased N2O, CH4 and CO2 emissions, probably due to reduced anaerobic conditions required for their synthesis, and led to a reduction of CO2 equivalents emissions.