The study, done by UNICEF, WHO and health organizations, said the rate in Vietnam fell to 8.2 percent in 2015 from 9.2 percent in 2000.

The number of newborns of below average weight, described as less than 2.5 kilograms or 5.5 pounds, stayed almost constant at 130,000 per year during the period.

Rana Flowers, UNICEF’s representative in Vietnam, said: "This figure is most encouraging. However as a national average, it does not give us the complete picture and is likely to hide important disparities."

She said among poor people 28 percent of mothers do not have institutional deliveries, and their children’s weights are not usually recorded.

After assessing birthweights in 148 countries and reviewing 281 million births, the study, the first of its kind, found more than 20 million newborns in 2015, or one in seven, had low birthweight.

Around 80 percent of all newborns who die have low birthweights.

Worldwide, the rate of low birthweight babies fell from 17.5 percent in 2000 to 14.6 percent in 2015.

The Philippines had the highest rate in Southeast Asia, 20.1 percent, followed by Laos (17.3), Myanmar (12.3) and Cambodia (12.1). East Timor's data was not listed.

The study said major causes of low birthweight are maternal malnutrition; maternal health problems such as high blood pressure, diabetes, and infection; maternal characteristics such as low/high maternal age, multiple parity, and poor birth spacing; and other risk factors such as smoking, alcohol and medically unnecessary caesarean deliveries. 

Babies with low birthweight have a higher risk of stunting, lower IQ and death during childhood, and experience overweight and obesity, heart disease and other noncommunicable diseases during adulthood, it found.