Abstract. A total of 984 primiparous and multiparous crossbred sows (Swedish Landrace × Large White) housed on a commercial pig farm were used to study the effect of the decrease in backfat thickness during lactation and the level of backfat at weaning on weaning-to-oestrus interval and conception rate of sows. Primiparous sows (n=213) received a single injection of 400 IU eCG + 200 IU hCG (PG600®, Intervet) at weaning and approximately 30 % of sows did not respond to the first treatment. First-farrowing sows produced significantly fewer live-born piglets, had markedly thinner backfat in late gestation, showed significant loss of backfat thickness during lactation and showed thinner backfat at weaning than sows with a greater number of parities. Primiparous sows with backfat thinner than 18 mm at weaning lost an average of 21 % of backfat during lactation and had a significantly longer weaning-to-oestrus interval (31.75 ± 2.22 days), even after a second treatment with PG600®, than both primiparous sows which responded to the first treatment (5.95 ± 0.16 days) and multiparous sows which were not treated (5.19 ± 0.12 days). The primiparous and multiparous sows with thicker backfat at farrowing also had thicker backfat at weaning (r=0.874 and r=0.938 for primiparous and multiparous sows, respectively). Sows with thicker backfat at weaning showed a shorter weaning-to-oestrus interval and this correlation was higher for primiparous than for multiparous sows (r=−0.192 and r=−0.100, respectively). Thicker backfat of the sows at weaning was moderately but significantly correlated with lower loss of backfat during lactation (r=−0.179 and r=−0.273 for primiparous and multiparous sows, respectively). The present study showed that monitoring of backfat thickness and loss of backfat during lactation represents a useful tool to decrease non-productive days and improve the efficiency of high-producing pig herds.