Introduction: Seasonal fertility is characterised by variation in reproductive performance of sows across seasons e.g. (King, 2017), farrowing rate is a key trait affected by seasonal changes with implications for the maintenance of production value; however, impacts of season on litter size are less clear. Sow thermal response to heat stress has been observed to differ between first parity and multiparous sows, with increased metabolic rate in primiparous sows as a consequence of growth and first lactation pressures (Gourdine et al., 2017). Consequently, first parity sows display a reduced farrowing rate from summer matings (Bunz et al., 2019). It is therefore hypothesised that litter size will be affected by season, and the magnitude of this change will be larger for younger sows.