A newborn baby was found to have an extremely rare condition that saw him needing surgery at just three days old.

A pregnant 32-year-old woman was discovered to have the rare congenital anomaly, which affected her then-unborn baby.

The unnamed mom had a scan at 35 weeks as part of a regular health check-up, which revealed that she had fetus-in-fetu - a condition so rare that it only occurs in about one in 500,000 live births.

As per the International Society of Ultrasound in Obstetrics & Gynecology (ISUOG), fetus-in-fetu is 'an extremely rare anomaly of monochorionic, diamniotic twins in which a malformed fetus resides in the body of its normal sibling during development'.

It's thought that there's only 200 recorded cases of this happening across the globe since medical records began, says NDTV.

The woman had the scan on February 1 and has since given birth to a baby boy.

The baby then underwent surgery a few days later to remove the fetuses from his stomach at a hospital in Amravati district of Maharashtra, India.

Dr Usha Gajbhiye was the one to operate on the baby boy, and later confirmed that the fetuses he had inside him appeared to be twins that have both developed hands and legs.

The fetuses were successfully removed and both the mom and son have been recovering since.

Talking to reporters after the procedure, Dr Agarwal spoke about the rarity of the situation and revealed that they were only aware of 10-15 cases of fetus-in-fetu in India.

"I was lucky and vigilant enough to notice something very unusual with this baby, grossly normal growing fetus with a few bones and a fetus-like structure in its abdomen," the doctor said.

"It struck me immediately that this is not normal. It was fetus-in-fetu, one of the rarest cases in the world. We asked for a second opinion and the case was confirmed by radiologist Dr Shruti Thorat."

As to what causes fetus-in-fetu, one theory is that it's 'a highly differentiated form of a teratoma (an abnormal collection of cells)', says ISUOG.

Another theory is the parasitic twins. Cleveland Clinic explains that this is 'a type of conjoined twin where one fetus stops developing but remains attached to its twin'.

"The other twin continues to develop, but is usually born with the limbs, organs or other tissue structures from its parasitic twin still attached. It’s a very rare condition," the website further details.