Background: Infant rapid weight gain (RWG) may predict subsequent obesity, but there are inconsistencies in the growth references and age intervals used for assessment.

Methods: This study evaluated whether the prevalence of RWG (an increase of >0.67 in weight-for-age z-score) differed by growth reference (2006 WHO standards vs 2000 CDC references) and age interval of assessment (0–3, 0–6, 6–12 and 0–12 months). Pooled data from singleton term infants from two observational studies on maternal mood disorders during pregnancy were used (n = 161). Differences in RWG prevalence by growth reference and age interval were tested using Cochran’s Q and McNemar’s tests.

Results: The CDC reference produced a higher RWG prevalence (14% of infants additionally categorized as RWG, p < 0.0001) within the 0–3 month age interval compared to the WHO standards; this pattern was reversed for the 6–12 and 0–12 month intervals. RWG prevalence did not differ across age interval within the WHO standards, but did differ with the CDC references (range: 22% for 0–3 months to 4.2% for 6–12 months, p < 0.0001).

Conclusions: Caution is advised when comparing studies with different criteria for RWG. Future studies should use the 2006 WHO standards and a consistent age interval of evaluation.