BACKGROUND: No consensus definition of macrosomia currently exists among researchers and obstetricians. We aimed to identify a definition of macrosomia that is more predictive of maternal and perinatal mortality and morbidity in low- and middle-income countries. METHODS: We conducted a secondary data analysis using WHO Global Survey on Maternal and Perinatal Health data on Africa and Latin America from 2004 to 2005 and Asia from 2007 to 2008. We compared adverse outcomes, which were assessed by the composite maternal mortality and morbidity index (MMMI) and perinatal mortality and morbidity index (PMMI) in subgroups with birthweight (3000-3499 g [reference group], 3500-3999 g, 4000-4099 g, 4100-4199 g, 4200-4299 g, 4300-4399 g, 4400-4499 g, 4500-4999 g) or country-specific birthweight percentile for gestational age (50(th)-74(th) percentile [reference group], 75(th)-89(th), 90(th)-94(th), 95(th)-96(th), and >/=97(th) percentile). Two-level logistic regression models were used to estimate odds ratios of MMMI and PMMI. RESULTS: A total of 246,659 singleton term births from 363 facilities in 23 low- and middle-income countries were included. Adjusted odds ratios (aORs) for intrapartum caesarean sections exceeded 2.0 when birthweight was greater than 4000 g (2.00 [95% CI: 1.68, 2.39], 2.42 [95% CI: 2.02, 2.89], 2.01 [95% CI: 1.74, 2.33] in Africa, Asia and Latin America, respectively). aORs of MMMI reached 2.0 when birthweight was greater than 4000 g, 4500 g in Asia and Africa, respectively. aORs of PMMI approached to 2.0 (1.78 [95% CI: 1.16, 2.74]) when birthweight was greater than 4500 g in Latin America. When birthweight was at the 90(th) percentile or higher, aORs of MMMI and PMMI increased, but none exceeded 2.0. CONCLUSIONS: The population-specific definition of macrosomia using birthweight cut-off points irrespective of gestational age (4500 g in Africa and Latin America, 4000 g in Asia) is more predictive of maternal and perinatal adverse outcomes, and simpler to apply compared to the definition based on birthweight percentile for a given gestational age.