Background Low birth weight (LBW) affects 6.9% of all UK births and has remained largely unchanged for many years. The United Nations and theWorld Health Assembly have set targets to substantially reduce global incidence. Understanding the contribution of modifiable risk factors to the burden of LBW is required to ensure appropriate interventions are in place to achieve this reduction. Methods Data from published studies on the risks from key modifiable factors were used alongside prevalence data from theWelsh population to calculate the population attributable risk for each factor individually and in combination. Results Fourteen risk factors accounted for nearly half of LBW births, and 60% of those to younger mothers (,25 years). Tobacco smoke exposure was the largest contributor.We estimated that smoking in pregnancy was a factor in one in eight LBW births, increasing to one in five for women aged under 25. Conclusions Risk factors are interrelated and inequitably distributed within the population. Exposure to one factor increases the likelihood of exposure to a constellation of factors further increasing risk. Action to address LBW must consider groups where the risk factors are most prevalent and address these risk factors together using multi-component interventions.