Associated Press: Surgeons in Nashville have separated conjoined twins.

It was the first time such a surgery has been performed at the Monroe Carell (KAIR'-uhl) Jr. Children's Hospital at Vanderbilt.

Keylee Ann and Zoey Marie Miller were born in January and immediately transferred to a neonatal intensive care unit. They are in critical condition.

Today's surgery lasted eight hours. The girls were fused from the lower breastbone to the navel. 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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