The feeding of high fibre diets to sows prior to ovulation has been shown to have beneficial effects on embryo viability, leading to a possible increase in piglet litter size. This trial was conducted to look at the effect of feeding sows a high fibre diet from mid lactation until breeding on subsequent litter size on a commercially run farm. The sows used were either Large White or Large White x Landrace in genotype and ranged from parity 1 to 7. The sows were allocated to receive either a cereal-based control diet (C), or a high fibre diet (HF) of similar specification but containing unmolassed sugar beet pulp (USBP) at a 20% inclusion rate during lactation and 40% from weaning to oestrus. The diets were fed from day 11 of lactation until oestrus. The sows were inseminated at oestrus and then fed a standard gestation diet until farrowing. The trial was conducted over a 3-month period and in total 198 sows received the HF diet with the rest of the sows over the 3-month period acting as controls (496) on the cereal-based diet. The effect of the diet on total litter size and the number of piglets born alive per sow was analysed in a general linear model (Minitab release 12.1). The combined results for both breed types showed that sows fed the HF diet had a significantly higher number of piglets born (12.37 ± 0.27 versus 11.41 ± 0.26, P<0.01) and a higher number of piglets born alive (11.47 ± 0.26 versus 10.85± 0.26, P<0.01) compared to 130 control fed sows matched for farrowing week. The difference in piglet litter size was still apparent when all the control sows farrowing over the 3 month trial period were included in the statistical analysis, with values of 12.47 ± 0.27 versus 11.79 ± 0.15 (P<0.05) for total number of piglets born and 11.77 ± 0.26 versus 11.16 ± 0.14 (P<0.05) for the number of piglets born alive for the HF and control diet respectively. Based on the results from this trial and previous studies, feeding a diet with a high content of USBP during late lactation and prior to insemination can increase the total number of piglets in the litter and the number of piglets born alive. This effect is less marked in crossbred sows with high baseline performance.