Almost three-quarters of these babies were born in Southern Asia and sub-Saharan Africa, where data are most limited.
In low-income countries, poor growth in the womb is a major cause of low birth-weight, according to the new analysis.
The study was undertaken by researchers from the London School of Hygiene & Tropical Medicine, UNICEF, and the World Health Organization (WHO), involving 148 countries and 281 million births, published in The Lancet Global Health journal.
In more developed regions, low birth-weight is often associated with prematurity (a baby born earlier than 37 weeks gestation), according to Unicef press release issued on Friday.
However, the problem also remains substantial in high-income countries in Europe, North America, and Australia and New Zealand, where there has been virtually no progress in reducing low birth-weight rates since 2000,
In 2012, all the 195 member states of the WHO committed to a 30 percent reduction in low birth-weight prevalence by 2025, compared with 2012 rates.
The estimates, which are the first of their kind, found that worldwide low birth-weight prevalence fell slightly from 17.5 percent in 2000 (22.9 million low birthweight livebirths) to 14.6 percent in 2015 (20.5 million).
However, the study indicates that at the current rate of progress—with a 1.2 percent yearly decline in low birthweight rates between 2000 and 2015—the world will fall well short of the annual reduction rate of 2.7 percent required to meet the WHO target of a 30 percent reduction in prevalence between 2012 and 2025.
These findings highlight the urgent need for more investment and action to accelerate progress, through understanding and tackling key drivers of low birthweight throughout life—including extremes of maternal age, multiple pregnancy, obstetric complications, chronic maternal conditions (eg, hypertensive disorders of pregnancy), infections (eg, malaria), and nutritional status, as well as exposure to environmental factors such as indoor air pollution, and tobacco and drug use.
“Despite clear commitments, our estimates indicate that national governments are doing too little to reduce low birthweight. We have seen very little change over 15 years, even in high-income settings where low birthweight is often due to prematurity as a result of high maternal age, smoking, medically unnecessary caesarean sections and fertility treatments that increase the risk of multiple births.
These are the underlying issues that governments in high-income countries should be tackling,” says lead author Dr Hannah Blencowe from the London School of Hygiene & Tropical Medicine, UK.
“To meet the global nutrition target of a 30 percent reduction in low birth-weight by 2025 will require more than doubling the pace of progress.”
The study authors call for international action to ensure that all babies are weighed at birth, to improve clinical care, and to promote public health action on the causes of low birth-weight to reduce death and disability.
“Every newborn must be weighed, yet worldwide, we don’t have a record for the birth-weight of nearly one third of all newborns,” says co-author Julia Krasevec, Statistics & Monitoring Specialist from UNICEF.
“We cannot help babies born with low birth-weight without improving the coverage and accuracy of the data we collect. With better weighing devices and stronger data systems, we can capture the true birthweight of every baby, including those born at home, and provide better quality of care to these newborns and their mothers.”
More than 80% of the world’s 2.5 million newborns who die every year are low birthweight because they are either born preterm and/or small for gestational age.
Low birthweight babies who survive have a greater risk of stunting, and developmental and physical ill health later in life, including chronic conditions such as diabetes and cardiovascular disease.
In this study, the researchers performed a comprehensive search of the available data from national government databases and national surveys to estimate prevalence and track trends on low birth-weight for livebirths in 148 countries from 2000 to 2015.
In total, data were collated from over 281 million births. However, the authors note that 47 countries (including 40 low- and middle-income countries that account for almost quarter of all births worldwide) had insufficient data available.
One of the lowest rates of low birth-weight in 2015 was estimated in Sweden (2.4pc). This compares to around 7 percent in some high-income countries including the USA (8pc), the UK (7pc), Australia (6.5pc), and New Zealand (5.7pc).