The 9-month-old girls were listed in serious but stable condition Thursday evening in an intensive care unit at Children's Hospital Los Angeles. Surgeons began the nearly 24-hour operation Wednesday morning, delicately separating a number of the girls' internal organs.
The identical twins were each born with one normally developed leg, and they shared a conjoined leg. Doctors used that leg to replace tissue and bone in the girls' pelvic regions.
Lead surgeon James Stein said he was pleased with the operation.
"For the girls, it's the beginning of a life with two separate bodies able now to participate in society as two individuals," he said.
The large intestine could not be divided and went to one twin. Surgeons also performed a liver separation and did reconstruction on the girls' urinary and reproductive tracts.
Doctors declined to identify the babies or their parents, citing the family's request for privacy.
The girls were born with a fraternal triplet sister who developed normally. The case marks the second known instance of conjoined twins born as part of a triplet set, according to the hospital.
The operation comes roughly a year after Guatemalan twins born joined at the head were successfully separated at UCLA's Mattel Children's Hospital.
Identical twins are born conjoined when embryos from a single egg fail to fully separate.
Of the approximately 200 pairs of conjoined twins born alive each year, about half die before their first birthday, according to Dr. Marcelo Cardarelli, University of Maryland Medical Center surgeon who helped separate twin girls from Uganda last year.