PHILADELPHIA, PA -- Two formerly conjoined twin girls from Mooresville, North Carolina are thriving months after going through separation surgery.

The Children's Hospital of Philadelphia (CHOP) released an update on Abby and Erin Delaney saying they are now able to sit independently from one another, be held separately and are practicing rolling over and crawling.

The now 15 month old girls underwent a successful separation surgery in June. The surgery was done in Philadelphia and took 30 doctors and nurses to complete the 11-hour operation.

"Nearly five months after separation, we are happy to announce that both Erin and Abbey Delaney are doing well as they continue to recover from this very complex surgery," said neurosurgeon Gregory Heuer, MD, PhD.

“This is one of the earliest separations of craniopagus conjoined twins ever recorded,” said Dr. Taylor. “We know that children heal better and faster the younger they are, therefore our goal for Erin and Abby was separation as soon as possible with minimum number of surgeries.”

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The twins were born in CHOP's Garbose Family Special Delivery Unit and have spent their entire lives living in the hospital until recently when Erin was discharged, the hospital said. Abby still remains in the hospital.

The twins were born 10 weeks premature, weighed two pounds each and were conjoined at the skull. Their chance of survival was estimated to be somewhere between five and 25 percent.

"Although this has been a long journey, with many ups and downs, Riley and I are thrilled to see how well the girls are doing today," said the twins' mother, Heather Delaney.

According to CHOP, over the next few years, the twins will need additional plastic and reconstructive surgery to replace the missing bone areas at the tops of their heads and to normalize their hairlines and minimize scarring. For now, after spending more than a year at CHOP, their parents are preparing to take them home to North Carolina sometime later this year.