FARGO (AP) - Being a conjoined twin has its benefits. When Isabelle and Abbigail Carlsen received their regular immunizations for 4-month-olds, the girls shared the shots - two into Abby's leg and two into Belle's. Still, the twins will have the full protection of seven vaccinations - just as if they had each received the standard four shots.

The Carlsen girls are joined at the abdomen and share parts of their livers and intestines. They also share blood flow through those organs.

Within less than a minute, anything in one girl's bloodstream flows to her sister, said Dr. Philip Fischer, medical director at the Mayo Eugenio Litta Children's Hospital in Rochester, Minn. The girls will have their separation surgery there May 12.

Knowing how their blood flows and how vaccinations work, a team of Mayo doctors determined they only had to give the two girls one dose of the vaccinations.

"It saved them twice as many shots and the double dosing, which could make them have more fever and achiness," Fischer said.

In the big scheme of things, vaccinations are a small part of the hurdles faced by the conjoined twins from Fargo.

But the situation also demonstrates how delicately intertwined the babies' lives are. Those details guide decisions from how much anesthesia and pain reliever the girls are given to how doctors eventually will separate them.

Doctors estimate the separation surgery will take at least 10 hours and could take as long as 14 hours. More than 18 doctors with specialties ranging from plastic surgery and pediatric cardiac surgery to vascular surgery will be involved.

The girls' parents, Amy and Jesse Carlsen, will receive periodic updates while they're with family and friends in a private room at the hospital.

Conjoined twins are rare, occurring once or twice for every 100,000 live births. Most experts believe they happen when a 2-week-old embryo destined for twins fails to separate completely.

There have been about 250 successful separations in which one or both twins have survived, according to the American Pediatric Surgical Association.

If there is such a thing, in some ways Belle and Abby are textbook examples of thoraco-omphalopagus twins. About 82 percent of these twins, who are joined at the chest and abdomen but have separate hearts, have been successfully separated, according to the APSA.

Like nearly all thoraco-omphalopagus twins, Abby and Belle share a pericardium - a plastic wrap-like sac around the heart. Their livers are fused.

The girls also share one duodenum, the first part of the small intestine. This is where food from the stomach mixes with bile from the liver and gall bladder and digestive juices from the pancreas. Doctors were briefly concerned about this because Belle wasn't growing as fast as her sister. Today, she has nearly caught up.

But if being a conjoined twin isn't rare enough, each case presents its own set of challenges. The Carlsen twins are no different. Unlike many conjoined twins, the Carlsen girls are offset. Belle is about a centimeter or two higher than her sister.

That complicates things for surgeons because the twins are interlocked: Their systems fit more like a jigsaw puzzle than two halves glued together.

"Usually with conjoined twins you can cut on the dotted line," said Dr. Christopher Moir, a pediatric surgeon who has been on Mayo teams that have separated two other sets of conjoined twins.

For example, Abby and Belle's fused livers overlap and sit at an angle, much like clasped hands. Part of Abby's liver is inside Belle and vice versa. Neither girl has a belly button, but both will get one during the surgery.

Even the girls' separate organs provide challenges. About one-fifth of Belle's heart is inside Abby's chest. Doctors don't believe the girls' hearts share any blood vessels, but they will be prepared if they find something different.

"We always have a 'what if' plan," said Penny Stavlo, a nurse practitioner with pediatric surgery. She coordinates the Carlsens' medical team.

One risk the girls face comes as surgeons settle the heart back into Belle's chest - they may kink the blood vessels. If that happens, the baby's blood pressure will immediately drop. In the worse-case scenario, surgeons will let the heart sink into place and close Belle's chest later.

Teams of doctors continue to study thousands of scans taken of the girls. Their understanding and the girls' outlook change by the day.

Despite numerous tests and scans, some of the girls' anatomy is still a mystery. Doctors still haven't gotten a good look at the girls' pancreases. The soft, spongy organs hide behind the duodenum that the girls share.

In March, doctors took x-rays of the girls' bowels. Each girl took turns drinking a bottle laced with barium to show contrast. Nine medical experts huddled around a TV screen to watch the milk flow through their systems, giving doctors a better view of their digestive system.

Recent tests highlighted the livers' blood supply and bile ducts - details that will prove crucial for a successful separation. The duct system keeps bile, which is needed to help break down fats, from backing up and destroying the liver.

The girls' livers have their own major blood vessels and bile ducts. But things get trickier from there.

Because the girls share part of the duodenum - into which the liver and pancreas drain - doctors will need to bypass that part of the bowel in one of the girls. They don't know yet who that will be.

Using what they call a Whipple procedure, doctors may suture parts of the pancreas, bile duct and stomach directly to the girl's intestine. The procedure is often done for patients with pancreatic cancer. It's seldom performed on children.

None of the procedures Abby and Belle will undergo is unique. All have been done before, although not necessarily under such circumstances, Moir said. That doesn't mean surgery will be simple.

"All this stuff we've seen before, just not all in one case," Moir said.

Still, doctors are optimistic the girls will survive. Surgeons recently told the parents the girls will have a 95 percent to 98 percent chance of making it through surgery, Jesse Carlsen said.

Whether Abby and Belle will have any long-term complications is more difficult to predict. The girls will spend three to 10 days in intensive care after surgery, Stavlo said. It will be longer before they can consider coming home. But one step at a time.