Smoking in pregnancy is a major determinant of low birthweight and a range of adverse infant health outcomes. There is a well-established social gradient in smoking in pregnancy in the US and northern Europe. Social gradients in health-related behaviours may result from longitudinal accumulation and cross-sectional clustering of social risk exposures. There is, however, no published confirmation of this explanation in empirical data with smoking in pregnancy as the outcome. This study aimed to test the effects of longitudinal accumulation and cross-sectional clustering of social risk exposures on smoking in pregnancy using data on the first pregnancies of 3163 female members of the 1958 British national cohort. Social class at birth and aged 11 years was used to create three dichotomous variables representing cumulative social class (both manual, one manual and one non-manual, both non-manual) early in the lifecourse. Cross-sectional clustering of social risk was represented by four dichotomous variables created from combinations of maternal age (<20 vs. 20+), own social class (manual vs. non-manual) and educational attainment (low vs. other). Cumulative social class in early childhood was associated with smoking in pregnancy in bivariate analysis but not after adjustment for cross-sectional clustering of social risk exposures. However, women who had been in the manual social groups at birth and 11 years were at increased risk of cross-sectional clustering of social risk exposures around pregnancy suggesting a pathway from early lifecourse risk exposure to social risk factors associated with a high risk of smoking in pregnancy. These findings suggest that the social gradient in smoking in pregnancy results from longitudinal accumulation and cross-sectional clustering of social risk exposures. Interventions aimed at reducing social inequalities in smoking in pregnancy need to account for cumulative and cross-sectionally clustered effects of social risk exposures.