A British mom became the first pregnant woman to undergo a surgery last summer to remove a “monster” tumor from her chest, the Telegraph reported.

Nicola Ellington, 26, and her then-unborn baby Layla Sky survived the two-hour surgery to rid Ellington of a teratoma tumor, which is characterized as a large tumor made up of hair and teeth. The watermelon-sized tumor was previously dormant, but began to grow because of pregnancy hormones, doctors said.

Dr. Dawn Adamson, a cardiologist at University Hospital Coventry and Warkwickshire NHS trust, first detected the tumor in Ellington’s chest.

"We think the hormones produced during pregnancy can trigger the growth of these types of rare tumors,” Adamson said. “None of us had ever seen anything like this before in pregnancy.”

The tumor is made up of embryonic cells, which, instead of developing a baby, form a tumor, Adamson said.

Before Ellington went to University Hospital Coventry, the tumor went undetected by two other hospitals and a general practitioner. Despite being in extreme pain, Ellington was told that her condition was normal due to pregnancy and was given morphine.

"The pain was so bad I could barely breathe and it felt like someone was pushing down on my chest,” Ellington said. “I was so worried about Layla Sky but doctors just kept telling me it was normal and sending me away with painkillers.”

But even after Adamson discovered the tumor, doctors were forced to wait to perform the surgery until the baby went through a period of vital development. Doctors decided it was time for the surgery when the tumor became so large that it was obstructing the flow of oxygen to the womb.

Dr. Joseph Marzouk of Britain’s University Hospital Coventry performed the surgery, which is the first of its kind in the world.

Layla Sky was then born in December weighing 7 pounds, 6 ounces.

"I remember after the operation I was shown the scans showing Layla was alright and the 'monster' had gone, I was so relieved,” Ellington said. “I have to thank the amazing doctors and surgeons at Coventry for the outstanding care and for saving not only my life but that of my unborn baby."

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