Dr Cass said she was healthy, adding: "Baby Boemer is still an infant but is doing beautiful."
He said the tumour occurs in one out of every 35,000 births.
The surgeon added: "Some of these tumors can be very well tolerated so the fetus has it and can get born with it and we can take it out after the baby's born.
"But about half of the time they cause problems for the foetus and it's usually causing problems because of a blood flow problem."