Researchers from the London School of Hygiene & Tropical Medicine, UNICEF and the World Health Organization analyzed data from government databases and surveys in 148 countries between 2000 and 2015.
The rates varied widely by region and by country, with 91% of low-weight babies born in low- and middle-income countries and three-quarters of all low-weight births occurring in sub-Saharan Africa and southern Asia.
In the United States, 8% of babies born in 2015 had low birth weight, compared with 7.5% in 2000.
High-income countries as a whole showed some of the slowest progress, with a combined low birth weight rate of 7% that persisted between 2000 and 2015, according to the study.
“Weight is the single most important factor about you at your birth that predicts your future health,” said Professor Joy Lawn, director of the London School of Hygiene & Tropical Medicine’s Maternal, Adolescent, Reproductive & Child Health Center and senior author of the new report.
Low weight at birth can occur when a baby is born prematurely or is born at full term but is small for his or her gestational age due to growth restriction in the womb, study co-author Dr. Mercedes de Onis of the World Health Organization said in a statement.
Babies with low birth weight have a greater risk of stunted growth, developmental delays and adult-onset conditions such as diabetes and cardiovascular disease, the authors explained.
In the United States, preterm birth is the major contributor to low birth weight, probably due to high rates of cesarean sections, the use of fertility treatments, high maternal obesity and maternal age. This in contrast to regions such as southern Asia, where most babies with low birth weight are born at full term but their growth in the womb has been restricted due to poor maternal nutrition, the researchers said.
“This is why reducing low birth weight requires understanding of the underlying causes in a given country,” de Onis said.
In 2012, the 195 WHO member countries pledged to reduce rates of low birth weights 30% by 2025. Between 2000 and 2015, the rate dropped 1.2% each year, according to the report.
In order to meet the 30% target by 2025, the speed of global progress will have to more than double, Lawn said.
To accomplish this, she says, efforts should focus on targeted prevention, on ensuring that every newborn around the world — regardless of whether they’re born at home or in a hospital — has a properly documented weight and on improving care for the 20.5 million babies with low birth weight.
“These new low birth weight estimates provide an opportunity to advance the agenda and call upon all stakeholders to take concerted action in the effort to ensure that every newborn is weighed at birth, and that the information is collated and used for local action and accountability at the household, community, district, national and global levels,” Professor Tanya Doherty of the South African Medical Research Council, who was not involved in the new research, wrote in an editorial published alongside the new study.