By Umberto Bacchi

LONDON (Thomson Reuters Foundation) - Improving mothers' nutrition before and during pregnancy is pivotal to reducing child stunting in developing countries, researchers said on Tuesday, as a new study showed poor child growth often starts in the womb.

Defined as low height-for-age, stunting affects one in three children in the developing world and carries severe, irreversible consequences for both physical health and cognitive function.

An analysis of data from 137 developing nations by a team of Harvard scientists found the leading cause of stunting is fetal growth restriction (FGR) - poor fetal growth in the womb resulting in a baby being abnormally small at birth.

Almost a quarter of a total of 44.1 million estimated cases among two-year-olds in 2010 were attributable to FGR, according to the study published on Tuesday.

Researchers said the findings called for "paradigm shift" from interventions focused solely on children to those also targeting mothers and mothers-to-be.

"It highlights the importance of developing a comprehensive intervention program to target moms and their families even they get pregnant in order to help their children's growth in the future," study co-author Kathryn Andrews told the Thomson Reuters Foundation in a phone interview.

Greater emphasis should be placed on ensuring that mothers have enough to eat and improving their diet with nutrient supplements, Andrews said.

FGR was already known to be one of numerous causes of stunting but the study was the first to rank each cause's relative contribution to the total number of cases, the authors said.

Poor sanitation and childhood diarrhea had the second and third largest impact after FGR, counting for 16.4 and 13.2 percent of cases respectively, according to the research, funded by the Canadian government through Grand Challenges Canada's "Saving Brains" program.

Other causes include infections, poor child nutrition and discontinued breastfeeding.

Last year the United Nations adopted an ambitious set of global development goals to end hunger and poverty by 2030.

Stunting affect both areas, as children who have poor growth in their first years of life tend to perform worse at school, which usually leads to poorer earning power later on.

"Knowing the major risk factors for stunting, the global cost of poor child growth, and the number of children missing developmental milestones are key pieces of information in ensuring children not only survive, but thrive," said Grand Challenges Canada's CEO, Peter Singer.

(Reporting by Umberto Bacchi @UmbertoBacchi, Editing by Ros Russell; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights and climate change. Visit http://news.trust.org)