The day before your infant daughters are going to be surgically separated is an unnerving one, according to Anna Clark.

"I'm a mess," she admitted. "I've been crying all morning. I'm excited, nervous. I couldn't sleep."

Her conjoined twin girls, Allyson and Avery, didn't have the same problem Monday. They were snoozing in their room at Primary Children's Medical Center and missed the press conference Monday afternoon where doctors and family talked about Tuesday's separation surgery.

The girls were born Oct. 20 in Los Angeles. Their father, Kerry Clark, who is an F-16 crew chief, transferred to Hill Air Force Base in January so the girls could be separated at Primary Children's.

The babies are ,joined at the very base of their spinal cords. Doctors say the spinal cords look like they connect in a U-shape, the joining very small and, hopefully, so low it will not be problematic for the girls in the future.

"Hopefully, the tissue joining them is not functional," said Dr. John Kestle, the neurosurgeon who's leading the twin-separation teams. But they cannot tell that from the imaging and won't know for sure until they operate.

How low on the spinal cord the twins are joined affects potential complications. If it's low enough, there are unlikely to be any. But even joined low, there's a risk one or both girls will have bladder, bowel or ankle problems. And the fact that the fluid sac surrounding their spinal cords must be opened creates a risk of leakage and meningitis. At their age — they'll turn 8 months one day after being separated — it's impossible to tell for sure how well their bowels and bladders already work. And complications may not be apparent until they are potty-trained.

Still, "it is not a long segment of spinal cord and we expect it to go great," Kestle said.

Kestle will have the challenge of separating the spinal cords while preserving the nerves. To do that, he said, he'll have to open the dura — the sac that surround the spinal cords — carefully tease the joined cords apart and then reconstruct the dura so that each infant has her own to protect her spinal cord.

Dr. Alan Stotts, an orthopedic surgeon, will be on hand during the surgery to separate a "small portion" of bone at the base of the tailbone that is also joined.

The closing up and "making it aesthetically pleasing" will be the job of plastic and reconstructive surgeon Dr. Faizi Siddiqi. Siddiqi weeks ago implanted tissue expanders —little balloons that are filled a bit at a time with saline — to stretch Allyson's and Avery's delicate skin so there will be enough to close wounds left by the operation.

Because they are so young and active and healthy, they're unlikely to need much in the way of physical therapy after the surgery, Stotts said, although they have not yet been able to attempt some milestones like crawling.

The care team will watch for other complications, as well. Spinal surgery can cause swelling and weakness that diminishes as that swelling goes down. It's also possible their spinal cords could get "stuck down" as the girls grow, but Kestle deemed that unlikely.

They'll also be watching for spinal curvature later, something that may be more likely for Allyson because she has a small piece of her vertebrae missing right above where the girls connect, Stotts said.

Because they have been lying in an unnatural position due to being conjoined, part of their skulls will still need some reshaping and they will continue to wear special helmets that do the reshaping for a couple of months after the surgery, Siddiqi said.

It's conceivable the girls could be safely tucked in their two separate cribs on base at Hill, where the family lives, in about a week.

If all goes as planned, this won't be so bad, said Kerry Clark. "Once they're standing up, it's going to get rough," he joked about having busy twins — especially since the family includes three other active little girls: Rylee, 6, Ryan, 4 and Karlee, 2.