Inadequate knowledge in maternal nutrition is one of the determinants of low birth weight. However, little evidence is available on whether maternal nutrition counselling alone can influence birth weight among women from low socioeconomic households. This study assessed the effect of prenatal maternal nutritional counselling on birth weight and examined the related risk factors. A cluster randomized controlled trial was conducted to assess the effectiveness of home-based maternal nutritional counselling on nutritional outcomes, morbidity, breastfeeding, and infant feeding practices by the African Population and Health Research Center in two urban informal settlements of Nairobi. The intervention group received monthly antenatal and nutritional counselling from trained community health volunteers; meanwhile, the control group received routine antenatal care. A total of 1001 participants were included for analysis. Logistic regression was applied to determine associations between low birth weight and maternal characteristics. A higher prevalence of low birth weight was observed in the control group (6.7%) than in the intervention group (2.5%; P<0.001). Logistic regression identified significant associations between birth weight and intervention group (adjusted odds ratio (AOR)=0.26; 95% confidence interval (CI), 0.10–0.64); maternal height <154.5 cm (AOR=3.33; 95% CI, 1.01–10.96); last antenatal care visits at 1st or 2nd trimesters (AOR=9.48; 95% CI, 3.72–24.15); pre-term delivery (AOR=3.93; 95% CI, 1.93–7.98); maternal mid-upper arm circumference <23 cm (AOR=2.57; 95% CI, 1.15–5.78); and cesarean delivery (AOR=2.27; 95% CI, 1.04–4.94). Nutrition counselling during pregnancy reduced low birth weight and preterm births, which was determined by women of short stature, early stoppage of antenatal visit, and cesarean delivery.