AbstractBackgroundThe INTERGROWTH‐21st sex and gestational age (GA) specific newborn size standards (IG‐NS) are intended to complement the World Health Organization Child Growth Standards (WHO‐GS), which are not GA‐specific. We examined the implications of using IG‐NS at birth and WHO‐GS at postnatal ages in longitudinal epidemiologic studies.ObjectivesThe aim of this study was to quantify the extent to which standardised measures of newborn size and growth are affected when using WHO‐GS versus IG‐NS at birth among term‐born infants.MethodsData from two prenatal trials in Bangladesh (n = 755) and The Gambia (n = 522) were used to estimate and compare size at birth and growth from birth to 3 months when using WHO‐GS only (‘WHO‐GS’) versus IG‐NS at birth and WHO‐GS postnatally (‘IG‐NS’). Mean length‐for‐age (LAZ), weight‐for‐age (WAZ) and head circumference‐for‐age (HCAZ), and the prevalence of undernutrition (stunting: LAZ < −2SD; underweight: WAZ < −2SD; and microcephaly: HCAZ < −2SD) were estimated overall and by GA strata [early‐term (37^0/7–38^6/7), full‐term (39^0/7–40^6/7) and late‐term (41^0/7–43^0/7)]. We used Bland–Altman plots to compare continuous indices and Kappa statistic to compare categorical indicators.ResultsAt birth, mean LAZ, WAZ and HCAZ, and the prevalence of undernutrition were most similar among newborns between 39 and 40 weeks of GA when using WHO‐GS versus IG‐NS. However, anthropometric indices were systematically lower among early‐term infants and higher among late‐term infants when using WHO‐GS versus IG‐NS. Early‐term and late‐term infants demonstrated relatively faster and slower growth, respectively, when using WHO‐GS versus IG‐NS, with the direction and magnitude of differences varying between anthropometric indices. Individual‐level differences in attained size and growth, when using WHO‐GS versus IG‐NS, were greater than 0.2 SD in magnitude for >60% of infants across all anthropometric indices.ConclusionsUsing IG‐NS at birth with WHO‐GS postnatally is acceptable for full‐term infants but may give a misleading interpretation of growth trajectories among early‐ and late‐term infants.