Earlier, couples eligible to have a second child, including those who are both only children and those from ethnic minorities, were asked to have their second child at least four years after their first.
But according to the new policy, couples eligible to have a second child can choose when they want to have it, head of Zhejiang Provincial Population and Family Planning Commission, Zhang Wenbiao, said.
He said the old policy did not help to tackle the unbalanced age structure of the population in Zhejiang, which is a major manufacturing and exporting base of China, the world's factory.
"Belated delivery may also be detrimental to the health of women and their babies as more women in urban areas are getting married in their thirties," he was quoted as saying by Xinhua news agency.
Over the next few decades, the province will see a rapid increase in its aging population and a steady decrease in the number of the population of working age, Zhang said.
The commission predicts that during the period of 2006 to 2011, about 177,000 babies are expected to be born earlier in the province, compared with those years when the "birth-interval" policy took effect.
But Zhang said it would not add to the province's gross population. The birth rate will remain around 1.5, within the country's requirement of 1.8, he said.