PHILADELPHIA—Surgeons have successfully completed operations to separate 8-month-old twin girls who were joined at the lower chest and abdomen, officials said.

Children's Hospital of Philadelphia said the seven-hour procedures to separate Allison and Amelia Tucker, of Adams, N.Y., were completed Wednesday afternoon.

Officials earlier said the twins, who shared their chest wall, diaphragm, pericardium and liver, were "excellent candidates" for separation. Lead surgeon Holly Hedrick said in a statement Wednesday night that the complex surgery "went very well and as expected."

The twins are recovering in the newborn/infant intensive care unit and will be monitored closely during their recovery, she said.

"We expect that, with this complex surgery behind them, Allison and Amelia will receive the care, therapy and support to allow them to live full, healthy and independent lives," Hedrick said.

The surgery was the 21st separation of conjoined twins performed at the hospital and involved a team of about 40 members, including physicians, nurses and other medical staff from general surgery, plastic and reconstructive surgery, cardiac surgery, anesthesiology, radiology and neonatology, the hospital said.

Conjoined twins occur once in every 50,000 to 60,000 births, and most are stillborn. About three-quarters of such twins are female and are joined at least partially in the chest, sharing organs. Chances of successful surgery and survival are greater if the twins have separate sets of organs, the hospital said.

A year ago, conjoined 20-month-old twins from the Dominican Republic were separated at a Virginia hospital. Maria and Teresa Tapia were attached at the lower chest, and surgical teams successfully divided their liver, pancreas and other shared organ systems and reconstructed their abdominal walls.

A few days before that, 2-year-old twin sisters Angelica and Angelina Sabuco, from San Jose, Calif., who were joined at the chest and abdomen, were separated in procedures performed at a California hospital.