"It won't have any impact on the issue of the aging society, but it will change the character of many young families," Wang said.
Too little, too late?
Under the 2013 reform, couples in which one parent is an only child were allowed to have a second child.
Critics said the relaxation of rules was too little, too late to redress substantial negative effects of the one-child policy on the economy and society.
Many couples who were allowed to have another child under the 2013 rules decided not to, especially in the cities, citing the cost of bringing up children in an increasingly expensive country.
State media said in January that about 30,000 families in Beijing, just 6.7 percent of those eligible, applied to have a second child. The Beijing government had said last year that it expected an extra 54,200 births annually as a result of the change in rules.
Chinese people took to microblogging site Weibo, China's answer to Twitter, to welcome the move, but many said they probably wouldn't opt for a second child.
"I can't even afford to raise one, let alone two," wrote one user.
Couples who flout family planning laws in China are, at minimum, fined, some lose their jobs, and in some cases mothers are forced to abort their babies or be sterilized.
William Nee, a China researcher at human rights campaign group Amnesty International, welcomed the move, but urged China to go further.
"China should immediately and completely end its control over people's decisions to have children. This would not only be good for improving human rights, but would also make sense given the stark demographic challenges that lie ahead," he said.
The plenum also announced plans to attack other structural economic challenges, covering areas such as market pricing, innovation, consumption and more private ownership of assets.