There is considerable interest in the impact of maternal exposures during pregnancy on birth outcomes. Clearly, exposures associated with poor birth outcomes need modification or avoidance. However, arriving at such estimates of association is made challenging by a number of features characteristic of the relevant data. First, exposures may be time varying (for example, cigarette and alcohol consumption) so that, to relate them to birth outcomes, one needs to model them and then extract derived parameters. Secondly, there are likely to be unequal numbers and spacings of exposure determinations during pregnancy. Thirdly, one needs to account for a variety of additional covariates. Finally, the variability and non-linearities inherent in birth outcomes mandate flexible modelling approaches. Here we use data from a cohort of East Boston mothers to assess the impact of smoking during pregnancy on birth weights. We emphasize modelling of, and then adjusting for, maternal weight gain during pregnancy and a proxy measure for pre-pregnancy weight, so as to obtain better estimates of the smoking effect. Throughout, our analysis is guided by appropriate graphics. The adjustment features an interesting application of varying-coefficient models. Results indicate that smoking related deficits in birth weights depend on the mode of adjustment, and that previously observed deficits of approximately 200 g are best recaptured with use of varying-coefficient models.