China on Saturday formally allowed couples to have a second child if one parent is an only child, the first major easing of the 3-decade-old restrictive birth policy.

Demographers and policy makers have estimated the easing would benefit some 15 million to 20 million Chinese parents — mostly in cities — and result in 1 million to 2 million extra births per year in the first few years, on top of the 16 million babies born annually in China. They say the easing is so incremental that the extra births are not expected to strain resources such as the health care and education.

China has credited the restrictive policy with managing its population growth and improving the economy, but critics say it is a violation of human rights.

China is the world’s most populous country with 1.35 billion people.