The objective of this study was to investigate whether maternal socio-economic status during childhood and at the time of pregnancy each have unique associations with infant birthweight when biological determinants of birthweight are controlled. The data are from a three-generation study which contains information on the mothers and grandmothers of 987 singleton infants, collected over a period of 25 years. We used simple and multivariable regression to assess the association between indicators of a woman's socio-economic status and her offspring's birthweight. Women who grew up in poor households had smaller babies than those who did not, and a unit increase in the income/needs ratio (analogous to the poverty index), in non-poor households only, was associated with a 185 g [95% CI 70, 200] increase in infant birthweight. Maternal age at the index infant's birth had a positive association with birthweight that diminished as women reached their mid-twenties. Among mothers with low education, high grandmaternal education was associated with a 181 g [95% CI 71, 292] increase in infant birthweight, while high grandmaternal education had no effect among infants whose mothers were relatively well-educated. This interaction between grandmaternal and maternal education is consistent with claims that cumulative stress is an important mechanism connecting maternal socio-economic status and infant health.