Conjoined twin girls who had been fused at the torso and shared a liver were separated Tuesday at a Tennessee hospital, officials said.

Keylee Ann and Zoey Marie Miller were born Jan. 4 in Johnson City. They were immediately transferred to the Neonatal Intensive Care Unit at Monroe Carell Jr. Children's Hospital at Vanderbilt.

The twins were cared for the last three months until they grew strong enough for Tuesday's 8-hour separation surgery. They are both in critical condition.

Parents Victoria Ford and Brian Miller are optimistic about their daughters' prognosis, according to a news release from the hospital.

Ford and Miller knew early in the pregnancy the twins were conjoined. They had hoped to carry them to term, but they were born by Cesarean section 10 weeks early in January.

While several sets of conjoined twins have previously been treated at Vanderbilt, this was the first separation surgery performed there.

The girls were fused from the lower breastbone to the naval. They shared a liver and were born with one umbilical cord.

Conjoined twins are identical and develop from the same fertilized egg. Their bodies are joined while in the uterus. In the U.S., the incidence for conjoined twins is 1 per 200,000 live births.

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