Background: Childhood obesity prevention interventions delivered by health professionals during the first 1,000 days show some evidence of effectiveness, particularly in relation to behavioural outcomes. External validity refers to how generalisable interventions are to populations or settings beyond those in the original study. The degree to which external validity elements are reported in such studies is unclear however. This systematic review aimed to determine the extent to which childhood obesity interventions delivered by health professionals during the first 1,000 days report on elements that can be used to inform generalizability across settings and populations. Methods: Eligible studies meeting study inclusion and exclusion criteria were identified through a systematic review of 11 databases and three trial registers. An assessment tool based on the RE-AIM (Reach, Effectiveness, Adoption, Implementation, Maintenance) framework was used to assess the external validity of included studies. It comprised five dimensions: reach and representativeness of individuals, reach and representativeness of settings, implementation and adaptation, outcomes for decision making maintenance and/or institutionalisation. Two authors independently assessed the external validity of 20% of included studies; discrepancies were resolved, and then one author completed assessments of the remaining studies. Results: In total, 39 trials involving 46 interventions published between 1999 and 2019 were identified. The majority of studies were randomized controlled trials (n=24). Reporting varied within and between dimensions. External validity elements that were poorly described included: representativeness of individuals and settings, treatment receipt, intervention mechanisms and moderators, cost effectiveness, and intervention sustainability and acceptability. Conclusions: Our review suggests that more emphasis is needed on research designs that consider generalisability, and the reporting of external validity elements in early life childhood obesity prevention interventions. Important gaps in external validity reporting were identified that could facilitate decisions around the translation and scale-up of interventions from research to practice. Registration: PROSPERO CRD42016050793 03/11/16.