Some 250 million children worldwide risk not reaching their full potential due to extreme poverty and stunting, cutting their future earnings by up to 26 percent and seriously impacting national growth, scientists said on Tuesday.

The failure to invest in early child development is costing some low and middle income countries two to three times their current expenditure on health, researchers calculated.

Experts say the first few years of life from conception onwards is critical for brain development.

Writing in The Lancet medical journal, the scientists highlight strong evidence linking stunting and extreme poverty to reduced cognitive and educational development, poorer adult health and lower earnings.

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New analysis of data from 2010 - the latest figures available - suggests 249.4 million or 43 percent of children under five in low and middle income countries risk not meeting their developmental potential - down from 279.1 million in 2004.

Factors include poor nutrition and sanitation, infections, lack of nurturing care and inadequate stimulation in the early years.

In sub-Saharan Africa, two thirds of children are estimated to be at risk of poor development due to stunting and poverty, followed by 65 percent in South Asia, and 18 percent in the Caribbean and South America, the researchers found.

The worst affected countries include Liberia, Zambia, Democratic Republic of Congo, Burundi, Niger and Malawi.

The series of reports in The Lancet comes ahead of a World Bank summit on Thursday focusing on the importance of investing in early child development for future economic growth.

Earlier this year World Bank President Jim Yong Kim warned that childhood stunting was a "great unrecognised disaster".

He said countries which failed to invest in early child development would be left behind because their future workforce would lack the intellectual capacity to compete in an increasingly complex, digital world.

Leaders and ministers from countries including Indonesia, Pakistan, Guatemala, Ivory Coast, Tanzania, Ethiopia, Madagascar, Cameroon and Senegal are set to attend the Washington meeting.

The authors of The Lancet reports highlight the importance of providing free pre-school education, parental leave to support bonding and childcare, support for breastfeeding and a minimum wage sufficient to lift families out of poverty.

Improved nutrition for mothers and young children, maternal mental health care and prevention of child mistreatment and violence also play a critical role, the authors say.

"Our economic analysis shows that the cost of inaction is huge, and in many countries far exceeds their spending on health," co-author Professor Gary Darmstadt of Stanford University School of Medicine said in a statement.

"Supporting nurturing care is a wise investment, and one that should be prioritised in all countries," added Darmstadt, a neonatal and child development expert.

The analysis shows the cost of inaction compared to government expenditure on health is particularly high in India, Pakistan and Madagascar.

In June a group of Harvard scientists estimated that children born in developing countries this year would lose more than $177 billion in potential life-time earnings because of delays in physical development.