China's top family planning body has warned couples that they must continue to obey the country's one-child policy until the law changes in March.
On Thursday, the government announced it would relax the rules to allow all couples to have two children.
It said the decision was made because of China's rapidly ageing population and to help support the economy.
However, officials have stressed that the one-child policy will continue to be enforced until the law is changed.
China's controversial one-child policy was introduced nationally in 1979 to slow the population growth rate, and is estimated to have prevented about 400 million births.
Though there were exceptions to the policy, most couples who violated it faced punishment, from fines and the loss of employment to forced abortions.
On Friday, a local official was quoted as saying that women pregnant with a second child would no longer be punished, suggesting that the new policy was already effective.
However, on Sunday, the National Health and Family Planning Commission said that local officials should continue to implement existing family planning laws until the two-child policy was ratified by lawmakers in March.
"The two-child policy must be implemented in accordance with the law," an official with the commission said in a
Until the new law was adopted, local officials "must seriously enforce existing policies" and "must not act of their own accord", the statement added.
The government estimates that 90 million couples will be eligible for the new two-child policy.
Correspondents say that despite the relaxation of the rules, many couples may opt to have only one child, as one-child families have become the social norm.
What was the one-child policy?
Introduced in 1979, the policy meant that many Chinese citizens - around a third, China claimed in 2007 - could not have a second child without incurring a fine
In rural areas, families were allowed to have two children if the first was a girl
Other exceptions included ethnic minorities and couples where both parents were only children
In 2013 the rules were further relaxed so that couples where only one parent was an only child could also qualify for a second child
Campaigners say the policy led to forced abortions, female infanticide, and the under-reporting of female births
It was also implicated as a cause of China's gender imbalance