She is scheduled for surgery on Monday in an effort to repair her diaphragm.
"There's still a lot in front of us," Murnaghan said, but then added: "Sarah's a fighter. She's always been a fighter."
The suburban Philadelphia girl initially received lungs from an adult donor after her parents sued over national rules that place children behind adolescents and adults on the priority list for adult lungs — even if the children are sicker and are capable of accepting adult organs.
The action spurred a national debate over the organ allocation process.
The Murnaghans and the family of 11-year-old Javier Acosta of New York City challenged the policy making children under 12 wait for pediatric lungs to become available or be offered adult lungs only after adolescents and adults on the waiting list had been considered. Both children have end-stage cystic fibrosis.
A federal judge ruled they should be eligible for adult lungs after HHS Secretary Kathleen Sebelius declined to intervene.
Janet Murnaghan said Sarah's condition began to "spiral out of control" shortly after the first surgery. A second set of lungs was found and transplanted even though the donor once had pneumonia, making the surgery extra risky. The second set was also from an adult donor.
The failure of the first transplant is not uncommon. A 2005 University of Pennsylvania study found nearly 12 percent of lung transplants experienced what's called primary graft failure, where the organ almost immediately begins to fail.
But the timing — she received a second set of lungs just three days after her first — was narrow.
Of 5,081 lung transplants performed between 2010 and 2012, there were only seven retransplants within a week of the initial operation, according to the United Network for Organ Sharing, the private nonprofit group contracted by the government to manage the transplant list.
"I think this very clearly illustrates that the decision needs to be scientific and medical, rather than judicial," said Lawrence O. Gostin, a health law professor at Georgetown University who questioned the legal basis of the rulings. "UNOS was under pressure from the publicity surrounding this case and the court's decision. It is highly unusual to get two transplants within days."
According to UNOS, a graft failure does not automatically propel someone to the top of the waiting list of potential recipients, who have been assigned scores based on need. But Sarah's new score made her eligible for the second set of lungs.
Earlier this month, the Organ Procurement and Transplantation Network, which is overseen by UNOS, resisted making rule changes for children under 12 seeking lung transplants, but created a special appeal and review system to hear such cases. The special review option will expire on July 1, 2014, unless the full board of directors votes to keep it in place.
Of the 1,663 people currently seeking a lung transplant in the U.S., 12 are between the ages of 6 and 10.