Doctors at the Jackson Memorial Hospital in Florida who carried out the landmark operation said the tumour was very rare it had been seen only once in 20 years at the hospital.
Tammy Gonzalez, who was 17 weeks pregnant at that time, saw a "bubble" coming out of her baby's mouth during a scan.
Tests confirmed the mass was a teratoma -- a large tumour made of different types of tissue -- ballooning from the soft palate of the foetus.
Two weeks later, Dr Ruben Quintero, director of the Fetal Therapy Center at the hospital, snaked a tiny camera and surgical tools through a quarter-inch incision in Gonzalez's growing belly and into the amniotic sac.
"I couldn't feel the incision because of the local anaesthetic, but I could feel the tube going into the sac," said Gonzalez, who was awake during the procedure. "It felt like a popping balloon," she was quoted as saying by ABC News.
A laser was then used to cut the peach-sized tumour from Leyna's lips. The operation lasted just over an hour and after five months, the baby was born.
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"When they finally severed the whole thing off and I could see it floating down, it was like this huge weight had been lifted off me and I could finally see her face," Gonzalez said, describing the surgeons as "saviours".
The baby, named Leyna, was born in October 2010 weighing over 3.5kg. She is now a healthy 20-month-old child. The only sign of her life-saving surgery is a tiny scar on her mouth.
Last week, Gonzalez returned to Jackson Memorial Hospital with Leyna to thank Dr Quintero for taking a chance. "It was very emotional. He is a lifesaver," she said.
But Quintero said Gonzalez should share the credit.
"She's grateful that we offered her this chance," he said. "But we could not have offered her the chance if she hadn't had the courage."
The details of the pioneering surgery was reported in the American Journal of Obstetrics & Gynecology.