The biological mother, Pavithra (23), a daily wage labourer from Ezhil Nagar, is married to Ganesan (25), who is unemployed and faces several criminal cases, police said.

Pavithra became pregnant with her second child in December last year and sought financial help from a neighbour, Dhanam. Dhanam suggested handing over the baby to a childless couple for money and introduced her to a private hospital nurse, Aandal.

Knowing that an Andhra Pradesh-based couple, Selvarasi and Gnanasekaran, were looking for a surrogate, Aandal proposed that they adopt Pavithra's child instead. The couple and Pavithra reached an oral agreement, under which the couple agreed to cover all pregnancy-related expenses and pay Pavithra Rs 4 lakh.

Last week, Pavithra was admitted to a private hospital in Villivakkam, where she delivered a baby girl on Tuesday. Following the delivery, hospital administrator Visalakshmi allegedly altered the medical records, replacing Pavithra's name with Selvarasi's to present the latter as the biological mother. Police sources said the hospital's doctor, Rajasekaran (45), was also involved in the fraud.

The illegal transaction came to light when neighbours noticed Pavithra returning home without her infant and alerted the police.

Following a probe, police arrested Pavithra (23); the buyer couple, Selvarasi (30) and Gnanasekaran (35); mediators Dhanam (50), Visalakshmi (45), and Aandal (45); and doctor Rajasekaran (40).

The child has been rescued and placed in a government home, police added.