"It was like running into a brick wall," he said in local press at the time. "It knocks the wind out of you. It was like being punched in the chest. And when Addison was [diagnosed], it was like being ejected from a car. You wonder, what's next?"
The phenomenon has only been recorded "a handful of times" in medical literature, according to Dr. Pooja Hingorani, a pediatric oncologist who treats Addison at Phoenix Children's Hospital.
"All cancer can happen in pregnancy," Hingorani told ABCNews.com last year. "But melanoma is the most common cancer to pass through the placenta from the mother."
About 30 percent of all mother-to-fetus cancers are melanoma, according to Hingorani, who said she has only seen four to five cases ever.
"When it is in the blood stream, it can go everywhere," she said.
Melanoma is a virulent form of skin cancer that begins in the cells that make the pigment melanin, but it can also begin in the eyes or intestines. According to the National Cancer Institute, about 76,000 new cases are diagnosed each year and 9,100 die of the disease.
Sun exposure is thought to be one of the causes of melanoma. Hingorani said cancer among women of childbearing age is on the rise, and those who are pregnant, should tell their doctors if they've had melanoma.
"After the birth, the placenta needs to be examined carefully," she said. "It's hard to say if we would have picked it up at birth, if Addison would have had a less extent of disease."
Meanwhile, Cox said he has been overjoyed with the medical care that Addison has received.
Cox said Addison's doctors hope to get her into clinical trials, if treatments start to fail.
"They got her in immediately when it was discovered, coordinated her care and are constantly looking forward to the next step."
"If Phoenix Children's had not been there," he said. "Addison would have already passed away."