A baby boy allegedly sold by the doctor who delivered him in China has been reunited with his parents, state media reported, in a case highlighting the problem of child trafficking.

Zhang Suxia, the doctor responsible for the birth in northwestern China's Shaanxi province, allegedly persuaded the parents to give up their child last month after informing them he had serious congenital diseases, the China Daily newspaper reported.

Zhang and two other suspects were detained, the report said, and police discovered the baby at the house of a farmer in the central province of Henan, where three more suspects were held.

The baby was reunited with his mother Dong Shanshan and father Lai Guofeng on Monday in Fuping, Shaanxi, the report said.

"Dong held him tightly as family members wept before the parents knelt to thank police for recovering the infant," the China Daily said.

Citing a police official in Henan, the paper reported that a farmer with three daughters bought the baby boy from the alleged traffickers for 60,000 yuan ($9,800).

Seven other families in Shaanxi have told police their newborns were taken away after Zhang told them their infants were sick and they would face heavy financial burdens as a result, it added.

Trafficking of children is a serious problem in China, blamed in part on the "one-child" policy which has put a premium on baby boys, with girls sometimes sold off, abandoned or put up for adoption.

Under the policy, aimed at controlling China's vast population of more than 1.3 billion, people who live in urban areas are generally allowed one child, while rural families can have two if the first is a girl.

In a much publicised case, Chinese police rescued 89 children and arrested 355 suspects in December after breaking up a series of child trafficking rings.