We examined the relationship between intrinsic and extrinsic variables and variation in size at birth and rates of postnatal growth in the big brown bat Eptesicus fuscus at maternity roosts in central Massachusetts and southern New Hampshire. Body mass and length of forearm at birth were lowest when precipitation in the month preceding births was relatively high. In addition, body mass at birth was significantly lower when ambient temperatures during late pregnancy were relatively low. Pups born to middle age mothers were heavier than pups born to young or old mothers. When evaluated using mark–recapture data (longitudinal method), postnatal growth in length of forearm and body mass was greater for pups born to middle age and older mothers and change in body mass was greater for female pups than for male pups. When growth data were evaluated using the trajectory method, the rate of growth for length of forearm was greatest when ambient temperature was highest. Age prediction equations were improved when maternal age was incorporated into predictive equations. We recommend considering both intrinsic and extrinsic variables in studies where maternal effort is evaluated and age prediction equations are used for estimating postnatal growth rates in free-ranging bats.