Doctors diagnosed her unborn daughter with a rare condition called "fetus-in-fetu". Picture: PxHere

At seven months pregnant, Mónica Vega discovered she had two umbilical cords inside her.

Doctors diagnosed her unborn daughter with a rare condition called "fetus-in-fetu" - where a malformed fetus is found in the body of its twin.

Fetus in fetu (or foetus in foetu) is a developmental abnormality in which a mass of tissue resembling a fetus forms inside the body, Wikipedia reported.

There are two theories of origin concerning "fetus in fetu". One theory is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other theory is that the mass is a highly developed teratoma - a tumour made up of several different types of tissue, such as hair, muscle, teeth, or bone.

"Fetus in fetu" is estimated to occur in one in 500 000 live births.

According to doctors working Vega's case, only one was baby was connected to her while the other was connected to her unborn daughter Itzmara, who had a mass that was her parasitic twin.

Insider reported that they were forced to perform an emergency C-section so that they could operate on the baby. They were worried that if they didn't remove the parasitic twin inside Itzmara, the mass could grow and injure the baby's organs.

Just 24 hours after delivering Itzmara, doctors performed a C-section on her as well to remove the mass. According to Mamás Latinas, the "foetus" had no heart or brain. 

Itzmara was in good health following the surgery and in the process of recovering, the publication reported.