We conducted a systematic review and meta‐analysis of population‐based cohort studies of maternal body mass index (BMI) and risk of adverse birth and health outcomes in low‐ and middle‐income countries. PubMed, Embase, CINAHL and the British Nursing Index were searched from inception to February 2014. Forty‐two studies were included. Our study found that maternal underweight was significantly associated with higher risk of preterm birth (odds ratio OR, 1.13; 95% confidence interval CI, 1.01–1.27), low birthweight (OR, 1.66; 95% CI, 1.50–1.84) and small for gestational age (OR, 1.85; 95% CI, 1.69–2.02). Compared with mothers with normal BMI, overweight or obese mothers were at increased odds of gestational diabetes, pregnancy‐induced hypertension, pre‐eclampsia, caesarean delivery and post‐partum haemorrhage. The population‐attributable risk (PAR) indicated that if women were entirely unexposed to overweight or obesity during the pre‐pregnancy or early pregnancy period, 14% to 35% fewer women would develop gestational diabetes, pre‐eclampsia or pregnancy‐induced hypertension in Brazil, China, India, Iran or Thailand. The highest PAR of low birthweight attributable to maternal underweight was found in Iran (20%), followed by India (18%), Thailand (10%) and China (8%). Treatment and prevention of maternal underweight, overweight or obesity may help reduce the burden on maternal and child health in developing countries.