Doctors in Virginia made science history on Monday when they successfully completed a first-of-its-kind surgical procedure to separate six-month-old conjoined twin girls.

The 14-hour operation was performed at the Children’s Hospital of Richmond at VCU in Virginia, where the girls are now in stable condition.

A’zhari and A’zhiah Jones of Franklin, Virginia, were born as so-called thoracopagus twins, meaning their bodies were fused at the abdomen and that they had heart abnormalities.

The first stage of the Jones twins’ phased separation began in October 2012, when surgeons divided their shared liver, and then closed the girls’ abdomens back up.

“As the girls became critically ill over the second week of their lives, we had to urgently separate their conjoined liver as this was the source of their uncompensated cross circulation,” Dr. David Lanning, surgeon-in-chief at the children’s hospital, said in a written statement. “However, complete separation at that time would almost assuredly have resulted in their deaths as A’zhari was in renal failure and A’zhiah had severe cardiac hypertrophy. A phased surgery was the optimal plan.”

In February 2013, surgeons placed tissue expanders in the twins’ abdomens, which enabled the growth of excess skin to be used for closure and reconstruction following surgery.

Such a phased procedure was groundbreaking, but this same medical team actually faced a similar situation before — they also separated 19-month-old twins Maria and Teresa Tapia in November 2011.

“While A’zhari and A’zhiah look similar to the Tapia twins from the outside, we were faced with a different set of challenges that we had to overcome as a team,” Lanning said in the statement. “Despite these differences, our experience with the Tapia twins has been vitally helpful with caring for the Jones twins.”

The Tapia twins were classified as omphalopagus, meaning they were fused at the liver and upper part of the small intestine, so each twin had her own heart and kidneys.

So when it came to the Jones twins: “Because more of their chests were involved than the Tapia girls, this made closure a bit more challenging,” Lanning told the Richmond Times-Dispatch about the Jones girls’ surgery. “The plastic surgery team really worked to get closure of the chest and the abdomen. Even though they are closed now, we do plan to take A’zhari back after the swelling is gone down a little bit to advance some of the chest muscle flaps a bit more to provide a little bit better coverage of her heart.”

But, how exactly may twins’ bodies become fused to begin with?

It happens when a fertilized egg divides as it would in cases of identical twins, but doesn’t fully separate. Conjoined twins occur in about one in 200,000 births, with around half being stillborn, according to The Guardian.

Case in point: Nachell Jones, the mother of A’zhari and A’zhiah, was 12 weeks pregnant when she learned she was having twins and then 13 weeks when doctors told her they were conjoined.

Hear more from Jones and the doctors in the press conference video above.