More than 20 million babies were born with low birthweight (less than 2500g; 5.5 pounds) in 2015\u2014around one in seven of all births worldwide according to the first-ever estimates documenting this major health challenge. These findings and more are documented in a new research paper developed by experts from the World Health Organization, UNICEF and the London School of Hygiene & Tropical Medicine, published in\u00a0The Lancet Global Health. More than 80% of the world\u2019s 2.5 million newborns who die every year are of low birthweight. Those low birthweight babies who survive have a greater risk of stunting, and developmental and physical ill health later in life, including diabetes and cardiovascular disease. \u201cLow birthweight is a complex clinical entity composed of intrauterine growth restriction and preterm birth,\u201d says co-author Dr Mercedes de Onis from the Department of Nutrition at WHO. \u201cThis is why reducing low birthweight requires an understanding of the underlying causes in a given country. For example, in Southern Asia, a large proportion of low birthweight babies are born at term but with intrauterine growth restriction, which is associated with maternal undernutrition, including maternal stunting. \u201cConversely, preterm birth is the major contributor to low birthweight in settings with many adolescent pregnancies, high prevalence of infection, or where pregnancy is associated with high levels of fertility treatment and caesarean sections (like in USA and Brazil). Understanding and tackling these underlying causes in high-burden countries should be a priority.\u201d Although close to three-quarters were born in Southern Asia and sub-Saharan Africa, the problem remains substantial in high-income countries in Europe, North America, Australia and New Zealand. High-income countries have seen virtually no progress. What is being done to tackle this major public health problem Reducing the incidence of low birth weight requires a comprehensive global strategy, which must include improving maternal nutritional status; treating pregnancy-associated conditions such as pre-eclampsia (hypertensive disease of pregnancy); and providing adequate maternal care, perinatal clinical services and social support. Affordable, accessible and appropriate health-care is critical for preventing and treating low birthweight. Reductions in death, illness and disability in newborn babies will only be achieved if pregnancy care is fully integrated with appropriate care for low birthweight babies.