Background
We sought to: (i) estimate the prevalence of significant cognitive delay (a marked delay in the development of general cognitive functioning) among nationally representative samples of young children in middle- and low-income countries; (ii) estimate the total number of children under 5 years of age with significant cognitive delay living in low- and middle-income countries; and (iii) estimate the potential impact of five preventative interventions.

Methods
Secondary analysis of data collected in Rounds 4 and 5 of UNICEF’s Multiple Cluster Indicators Surveys in 51 countries involving 163 293 3- to 4-year-old children. Adjusted population-attributable fractions were used to estimate the potential impact of five interventions based on Sustainable Development Goals (SDGs).

Results
The prevalence of significant cognitive delay in 3- to 4-year-old children in middle- and low-income countries was 10.1% (95% confidence interval 9.7–10.4%). Prevalence was strongly inversely related to country economic wealth. The estimated total number of children under 5 with significant cognitive delay living in low- and middle-income countries was just under 55 million. This number could be reduced by over 60% if three separate SDGs were achieved; every mother had secondary-level education, every household had access to improved water and sanitation, and every child had an acceptable level of home stimulation.

Conclusions
Our results provide additional evidence in support of a range of specific preventative interventions in early childhood to reduce the loss of developmental potential among children in low- and middle-income countries.