A baby in India was born "pregnant" with his half-formed twin in a rare case that has astonished the medical fraternity in the country.

Some mass with the presence of bones was discovered early July in the then fetus' abdomen during a routine scan of the 19-year-old pregnant mother, said radiologist Bhavna Thorat.

"In the post-natal scans, we spotted another half-formed baby with a brain, arm and legs in a fetal sac in the baby's abdomen," Thorat said over phone.

"The mother had conceived twins. But here a twin got enveloped in the body of another twin, leading to a condition of a host baby and its parasitic twin, who grew up to 13 weeks and then stopped," she said.

This was a rare case of fetus-in-fetu, a congenital anomaly in which a malformed and parasitic fetus grows inside the body of a baby, Thorat added.

After his birth on July 20 at Bilal Hospital in Thane, near the western city of Mumbai, the baby underwent surgery last week to have the fetus removed.

Pediatric surgeons at a separate hospital operated on July 25 to remove the unborn twin, a male about 7 cm in length and weighing around 150 grams.

"There were no complications. Both the mother and the baby boy are still admitted but are in good health," gynecologist Neena Nichlani told dpa.

Nichlani and Thorat said only 100 such cases have been documented worldwide. A fetus-in-fetu is estimated to occur once in every 500,000 live births, Thorat said.