**Background:** Despite overall improvements in development indices, one in five children in high-income countries lives in poverty. Socioeconomic disadvantage during fetal life and early childhood adversely shapes life-course health trajectories. This systematic review, part of the EU Horizon 2020 LifeCycle project, aimed to assess whether income-support interventions delivered during the first 1,000 days of life (pregnancy to age 2) can improve cardiovascular, metabolic, respiratory, and mental health outcomes in children.
**Methods:** The review included experimental or quasi-experimental studies from high-income countries (World Bank definition). Searches were conducted across multidisciplinary databases (SCOPUS, Web of Science, Google Scholar), social science databases (SSRN, Econlit, PsycInfo), biomedical databases (PubMed/Medline, EMBASE), and the Cochrane Library CENTRAL, between October 2020 and February 2021, updated August 2022. A snowballing approach was used. Two authors independently screened papers at title, abstract, and full-text stages. Data extraction covered general paper information, intervention characteristics, impact evidence, and study limitations. Due to heterogeneity, a narrative synthesis was performed rather than meta-analysis. Risk of bias was assessed using the Waddington et al. approach, identifying selected biases without applying a score-based approach. The protocol was registered in PROSPERO (CRD42020178543).
**Key Results:** The search returned 11,658 papers; after screening, 16 papers met inclusion criteria (15 quasi-experimental, 1 RCT). Publication years ranged from 2001 to 2019, with interventions delivered between 1957 and 2013. Most studies were from the United States and Canada; one was from Scotland. Interventions included: Earned Income Tax Credit (EITC, 7 papers), unconditional cash transfers (5 papers), conditional cash transfers (3 papers), and minimum wage policies (2 papers). The vast majority of studies focused on birth weight; only 3 examined child mental health. No studies investigated cardiometabolic or respiratory outcomes.
For birth weight, evidence was relatively consistent: most EITC studies found a decline in low birth weight (LBW) rates ranging from 1.6% (Hoynes et al.) to >10% (Komro et al. 2019). Komro et al. 2019 reported absolute birth weight increases from 8.6g (non-Hispanic women receiving non-refundable credit <10% of federal amount) to 37.1g (Black women receiving refundable credit >10% of federal amount). The same study estimated that EITC resulted in 3,760 fewer LBW babies from Black mothers and 8,364 fewer LBW babies from White mothers annually across the US. For minimum wage, Komro et al. 2016 estimated that a $1 increase would yield 2,790 fewer LBW births annually. Brownell et al. found an unconditional cash transfer in Canada reduced LBW risk (RR 0.71, 95% CI 0.63–0.81) and SGA risk (RR 0.90, 95% CI 0.81–0.99), but increased LGA risk (RR 1.13, 95% CI 1.05–1.23).
For mental health, results were mixed. The only RCT (Morris et al., SSP conditional cash transfer) found null effects. Two quasi-experimental studies (Hamad et al. 2016 on EITC; Milligan et al. on Canadian Child Tax Benefit) found positive effects on child mental health scores.
Null results were observed in three studies. The only study with a negative finding was Brownell et al. (increased LGA risk). All studies had at least one identified bias, most commonly exposure misclassification and incomplete reporting.
**Clinical Implications:** The review suggests that income-support interventions have a modest positive effect on birth weight, with limited impact on mental health. The authors note that despite small individual effects, the population-level impact may be substantial given the large number of beneficiaries. However, the evidence base is limited by: (1) heavy predominance of North American studies (limiting generalizability), (2) focus on only two health outcomes (birth weight and mental health), (3) reliance on quasi-experimental designs with only one RCT, and (4) potential confounding by historical trends in LBW and mental disorders. The authors call for more impact evaluations, mixed-methods approaches to understand mechanisms, and expansion of evidence to other health outcomes, countries, and intervention types. They emphasize that the strong association between lower income and poorer child health, contrasted with modest intervention effects, represents a 'research call' to better understand what works, for whom, and why.