More than 20 million newborns in 2015 -- one in seven -- came into the world weighing too little, according to a global assessment of birthweight, published Thursday.
Over 90 per cent of babies tipping the scale at less than 2.5 kilos (5.5 pounds) when born were in low- and middle-income countries, researchers reported in The Lancet Global Health.
Worldwide, just under 15 per cent of 2015 newborns in the 148 countries canvassed had low birthweight, varying between 2.4 per cent in Sweden and nearly 28 per cent in Bangladesh.
That's down from a global average of 17.5 per cent in 2000.
But meeting the World Health Organisation target of cutting low birthweight 30 per cent between 2012 and 2025 "will require more than doubling the pace of progress," said lead author Hannah Blencowe, a professor at the London School of Hygiene & Tropical Medicine.
In sub-Saharan Africa, the number of low birthweight live births actually increased from 2000 to 2015, from 4.4 to 5 million.
Southern Asia is estimated to have had 9.8 million in 2015, nearly half the world total.
Weighing less than 2.5 kilos at birth is closely linked to high rates of neonatal mortality and ill health later in life: more than 80 per cent of the world's 2.5 newborns who die every year are low birthweight.
Underweight newborns who survive also have a greater risk of stunting as well as developmental and health problems, including diabetes and cardiovascular disease.
"National governments are doing too little to reduce low birthweight," Blencowe said in a statement.
"To meet the global nutrition target of a 30 per cent reduction by 2025 will require more than doubling the pace of progress."
The reasons for low birthweight are very different in poor and rich regions.
In South Asia and parts of sub-Saharan Africa, a large percentage of underweight babies are born at term but are stunted because their mothers were undernourished.
In North America and Europe, a higher share of low birthweight babies are preemies.
Adolescent pregnancies, a high prevalence of infection, high levels of fertility treatment, and a high rate of caesarean sections -- especially in the United States and Brazil -- can all be factors, the study found.
An international team of researchers analysed national government databases to estimate the prevalence of low birthweight in 148 nations from 2000 to 2015.
Overall, the study took into account 281 million births. Several countries -- including India -- were not included for lack of data.
"Every newborn must be weighed, yet worldwide we don't have a record for the birthweight of nearly one third of all newborns," said co-author Julia Krasevec, a statistics and monitoring specialist at UNICEF.
Besides Sweden, other countries with relatively few low birthweight babies included Finland (4.1 per cent), Iceland (4.2) Serbia (4.5), Norway (4.5), Albania (4.6), China (5), Croatia (5.1) and Cuba (5.3).
Many large advanced economies fell in the 6-8 per cent range, including France, the United States, Britain and Germany, Mexico and Brazil.
Five countries, including Bangladesh, had low birthweight rates above 20 per cent: Comoros, Guinea-Bissau, Nepal, and the Philippines.
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