Low piglet weight at weaning implies a loss of income for the farmer and might influence the welfare of the affected animals negatively. The objective of our cohort study was to identify risk factors in the farrowing section associated with average daily weight gain (ADG) of piglets from birth to weaning. The study was done in 3 Danish farrow-to-finish herds and included 581 sows, 277 pens, 814 farrowings, and 8241 piglets. The data were analysed with a linear mixed model where the response was ADG, and sow and litter were random effects. Piglet treatments for arthritis (-38 g per day), diarrhoea (-8 g per day) and other infections (-21 g per day) were the major infectious contributors to lower ADG. Forelimb-skin abrasions on a piglet (-5 g per day), weak pasterns of sows on concrete slats (-34 g per day), suckling a sow with poor milking 3 weeks after farrowing (-14 g per day), low birth weight (-8.4 g per day per 100 g), male compared to female (-4 g per day) also were associated with lower ADG in the suckling period.