Last May, Silvia Hernandez gave birth to triplets: three little girls named Ximena, Scarlett and Catalina.

Catalina was in good health, but Ximena and Scarlett were conjoined -- attached at the hips -- a discovery doctors made even before the babies were out of the womb.

Delivered by C-section at Corpus Christi Medical Center, the Texas twins needed specialized care and were transferred to nearby Driscoll Children's Hospital, where they have lived since they were born on May 16, 2015.

Get Breaking News Delivered to Your Inbox

Their mom, who is 23, told CBS News that she and Catalina and her oldest child, 3-year-old son Raul, have been living in a nearby Ronald McDonald House during that time.

Hernandez, who speaks Spanish and some English, said the past year has been "difícil" -- difficult.

"For the moment, my mom is helping me to take care of my son and Catalina at the Ronald McDonald House and that way I can come to the hospital and visit and divide my time between both of them," Hernandez explained through an interpreter.

Ximena and Scarlett are connected below the waist -- fused at the hips, but they have separate lower limbs. They were born sharing a colon and some reproductive organs, including the uterus and ovaries. While they have two bladders, they do not function properly.

They take nutrition through a feeding tube, so Hernandez has not had the opportunity to feed her daughters yet.

The surgery is scheduled for April 12th at Driscoll. The operation is expected to last anywhere from 12 to 18 hours, according to the hospital, during which time the twins will be separated and undergo reconstructive surgery.

One out of every 200,000 births results in conjoined twins, but the chance of a triplet birth involving conjoined twins is extraordinarily rare -- perhaps one in 50 million, Driscoll specialists said. [CBS News chief medical correspondent Dr. Jon LaPook reported on a similar case in Haiti last year.]

The way these babies are connected is uncommon, said pediatric surgeon Dr. Haroon Patel, who heads up the team that will operate on the twins. So there is not a depth of medical literature on the type of separation they will perform, especially not modern references using technologies currently available.

"This arrangement is fairly rare, in only about 6 percent of conjoined twins," Patel told CBS News.

"The last time something was published like this in the U.S. was 1966," he said.

It will be a first-of-its-kind surgery for the hospital and experts in multiple specialties are joining ranks, including pediatric surgery, plastic surgery, urology, and orthopedic staff.

A team of specialists has been working for months to prepare for the complex surgery and they expect a successful outcome. Patel said he and colleagues draw from their experience with children who've had congenital abnormalities.

Three-dimensional CT scans and magnetic resonance imaging have helped doctors there create a 3D model of the twins' anatomy -- a virtual representation of the twins on a computer screen so that surgeons can simulate the procedure before they perform it in real life. The babies' surgeons will use the virtual technology as a reference during the operation, as well, according to the hospital.

"There will be skin and muscle separation, but once on the inside, we'll have to split the colon. The plan is to give half to one and half to the other," Patel explained.

Right now, the babies' kidneys go to the opposite baby's bladder, so the surgeon will need to "reroute" them to go to the correct organ. "We need to reroute the plumbing so each baby has the same kidney emptying into the same bladder," he said.

The surgical team will also use a new technology called the Image Perfusion Scanner. The scanner, or "Spy Camera," assesses blood flow in areas where physicians are operating and provides critical information to assist surgeons in determining where to divide skin and organs.

"The goal here is essentially separation and closure. I think most of that should be accomplished this operation," Patel said.

Ximena and Scarlett will remain in the pediatric intensive care unit at Driscoll Children's after the operation for specialized care, and should be able to head home within three months time. The twins will also require additional operations as they grow, though not on the scale of their upcoming surgery.

They developed a viral respiratory infection earlier this month so plans for their separation was pushed back another month, but they are healthy as they head toward the new April operation date, said Patel.

Dr. Miguel DeLeon, medical director of Driscoll's Neonatal Intensive Care Unit, said in a statement, "The babies have been doing very well as we've focused on getting them healthy for this complex procedure."

The twins' mother told CBS News that she is experiencing a mix of emotions as the surgery date nears.

"Since they were born, I have been waiting anxiously for them to be separated because I want to hold them separately in my arms and hold them close. But the closer the surgery day is getting, I don't want it to happen. But of course I want it to happen so they can have a normal life," said Hernandez.

"My daughters are in hands of God and his will will be done."