(CNN) Over 20 million babies around the world -- about 1 out of every 7 -- were born underweight in 2015, a slight improvement over rates in 2000 but not enough to meet goals and prevent global health consequences, according to a new study.

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.

Globally, 20.5 million -- 14.6% -- babies born in 2015 had low birth weight, defined as less than 2,500 grams or about 5.5 pounds. That's a slight decrease from the 22.9 million -- 17.5% -- babies with low birth weight in 2000, according to the findings, published Wednesday in the journal The Lancet Global Health

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.

Read More