But the mark - originally the size of a golf ball on the left hand side of Isabel's face - trebled in size within a month and covered her left ear.
"When she was born they (doctors and nurses) brought the baby over and said 'she has a small lump," Yvonne recalls.
"I thought 'that's not a small lump, it is massive'. I was crying, I had never seen anything like it before. It was just ginormous.
"It was supposed to be a happy day but it wasn't at all.
"They tried to reassure us and said it was a strawberry birthmark but it just did not look right."
An hour after her birth on March 1, weighing a healthy 7lb 10z after a planned Caesarean at Poole Hospital, Dorset, Isabel was taken for an ultrasound examination, from which doctors concluded the mark was a harmless birthmark.
"We must have seen 20 doctors and they never questioned the original diagnosis, they just all said 'it's a birthmark' and told us not to worry," says Yvonne.
As the mark began to grow and bleed, Yvonne, 32, a nurse herself and mother-of-four, repeatedly took Isabel back to hospital but she said doctors prescribed only a medication to shrink birthmarks.
Yvonne insisted on further investigation and Isabel underwent an MRI scan at Southampton Hospital on March 6.
She said: "The doctor was really nonchalant and said 'I bet on my life that it is a birthmark, I don't want to see you for a year'.
"We had to chase the results for about two to three weeks. Luckily, another doctor at the hospital saw them and we were referred to Great Ormond Street for a review."
But as the family waited for their April 19 appointment in London, Isabel's mark began to grow and trebled in size.
They returned to Poole Hospital on April 5 and stayed for four days. Doctors remained insistent that the tumour was a birthmark and prescribed propanolol to shrink it.
But the mark began to blister and, on April 16, it ruptured while Yvonne and her partner Anthony Roch, were giving Isabel a bath.
They returned to Poole Hospital and demanded that Isabel stay in for observation for the next three days until her appointment.
On April 19 Isabel was taken by ambulance to Great Ormond Street Hospital, where an ear, nose and throat consultant ordered a biopsy.
The biopsy was performed a day later. On April 24, Isabel's parents were given the devastating news that she had an extremely rare sarcoma tumour and the cancer had spread to her bone marrow.
Five days later on April 29 Isabel had a six-hour operation to remove the cancerous cells on April 29. She has lost her entire ear and has a large scar on her face.
Isabel, who is now four months old, began chemotherapy on May 1. Yvonne and Isabel's dad Anthony, 37, a recruitment consultant, have been told their little baby may need more chemotherapy in the future should the cancer return.
Yvonne said: "To consider what she has gone through in such a short space of time, she is amazing, absolutely amazing.
Yvonne is now considering legal action against the hospital.
A spokesman for Poole Hospital NHS Foundation Trust told the Daily Mail: 'We are very concerned to hear the family is unhappy with the care their daughter received, and would urge them to contact us directly so we can discuss these fully and answer any questions they may have.
'We have a duty to protect our patients' confidentiality and it would not be appropriate for us to comment further.'
A spokeswoman for University Hospitals Southampton NHS Foundation Trust declined to comment.