A multi-suckling (MS) system for sows and piglets has been developed aiming to improve animal welfare. In this system, large variation in BW gain exists between piglets up to weaning at 9 weeks of age. We aimed to study the causes of variation in BW gain and DM intake of solid feed (DFI) (piglet + sow feed) of piglets during lactation in the MS system. A total of 15 sows and 60 focal piglets across three batches were studied. Individual intake of piglet and sow feed was measured by the dual marker method, and multiple variables were recorded. Multiple linear regression analysis with forward selection was conducted on BW gain and DFI after correcting for piglet sex and batch, using multiple explanatory variables including genetic background, birthweight (BiW), DM feed intake, behaviours and number of skin lesions. These factors jointly explained less than 45 % and 21 % of the variation in BW gain and DFI, respectively. In weeks 2–4, variation in BW gain was mainly explained by BiW (12.0 %) and play and nosing behaviours (7.6 %). In weeks 4–6 and 6–8, it was largely explained by DM intake of piglet feed with 15.1 % and 25.9 %, respectively. Individual variation in DFI in weeks 2–4 was explained by the presence at front and middle teats during suckling bouts (2.9 %), in weeks 4–6 by BiW (9.6 %), and in weeks 6–8 by the number of skin lesions (5.1 %). The unexplained variation in BW gain and DFI warrants further investigation.