Maternal and child mortality rates are still unacceptably high in South Africa. The health status of women in peri-urban areas has been influenced by political and socio-economic factors. Examining socio-economic variables (SEV) in a population aids in the explanation of the impact of social structures on an individual. Risk factors can then be established and pregnant women in these higher risk groups can be identified and given additional support during pregnancy. The aim of this study was to investigate the association between SEV and gestational Body Mass Index (GBMI) in a peri-urban settlement, South Africa. This was a sub-study of the Philani Mentor Mothers’ Study (2009–2010). Maternal anthropometry and SEV were obtained from 1,145 participants. Multinomial regression was used to analyse the data. Household income was the only SEV that was significantly associated with GBMI. The odds of being underweight rather than normal weight during pregnancy increase by a factor of 2.145 (P < 0.05) for those who had a household income lower than R2000 per month. All other SEV were not significant. Logistic regression was therefore not carried out. Women who had a lower income were at risk of having a lower GBMI during pregnancy. This can lead to adverse birth outcomes such as premature birth, low birth weight, height and head circumference. Public health policy needs to be developed to include optimal nutrition health promotion strategies targeting women with a low income ante and post-natally. Once implemented, they need to be evaluated to assess the impact on maternal and child mortality