This study examines whether the association between social inequalities and low birth weight (LBW) (occurring in both pre- and full-term births) in Germany can be explained by several potentially confounding factors. These include maternal age, occupational status, marital status, nationality, employment status, smoking, prenatal care, psychosocial stress, obesity, short stature, short inter-pregnancy interval, chronic conditions, and several obstetrical risk factors such as pregnancy induced hypertension. We also examined how the risk for LBW varies over time within each socioeconomic group. We analyzed routinely collected perinatal data on singletons born in the federal state of Lower Saxony, Germany, in 1990, 1995, and 1999 (n = 182,444). After adjustment for all potentially confounding factors in multivariate logistic regression models, working class women, unemployed women, single mothers, and women over 39 years of age were at increased risk for pre- and full-term LBW infants. Migrant status was not related to LBW. We examined variations in the risk for LBW over time within groups, using the 1990 birth cohort as the referent group for the 1995 and 1999 birth cohorts. Compared to 1990, in 1999 women aged 19-34 years, housewives, unemployed women, women of German nationality and women with partners had higher risks for pre- and full-term LBW infants; the eldest subgroup had lower risks for LBW after adjustment for confounding factors. The factors we examined partly explain the social inequalities in LBW occurring in pre- and full-term infants. The subgroups with higher rates of LBW in 1999 compared to 1990, included women experiencing childbirth in an optimal stage of life or in a privileged social context. Public health policies in Germany should target social inequalities contributing to the aetiology of LBW and to the factors that result in increased LBW rates.