The science of conjoined twins has intrigued the world for hundreds of years. The exact causes of the phenomenon are still unknown. Sixty percent of conjoined twins are stillborn and 40 percent of the live births die within a few days.
When they survive, each twin deals with a unique and delicate physiology.
In addition, the discovery of conjoined twins can be overwhelming for parents.
Michelle and Jeff Roderick learned she was carrying thoracopagus, or twins joined at the chest, during her 1996 pregnancy.
"When I found out that the girls were going to be born conjoined, it was really a scary time," Michelle Roderick said.
Pediatric surgeon James Stein said the most severe form of thoracopagus twins is when the babies actually share a whole heart.
"Obviously there's no way for both twins to survive if you were to attempt separation," he said.
But for the Rodericks, MRI images showed their girls didn't share the vital organ, which was a relief to them.
Their daughters were relatively good candidates for separation surgery because they had two separate hearts.
Even before the conjoined twins were separated, they struggled.
"Shawna wasn't doing so good. She was slower to come around," Jeff Roderick said.
Michelle Roderick said Shawna was smaller than her sister and had trouble.
"Janelle, in this case, would get really mad," Michelle Roderick said. "She'd start just really throwing a fit. And when she cried so hard, she would actually push the surface blood off of her body and into Shawna's body. And Shawna wasn't crying, and all of a sudden she just would turn beet red, and Janelle looked really scary white."
The Rodericks and doctors felt the twins should be separated as soon as possible, at just 30 days old.
The girls were conjoined in three places: their breastbones, livers and diaphragms. While the bones and diaphragm are simple to separate, the shared liver presented the most difficult challenge.
Doctors had to carefully split the liver in half. Because the liver is the body's only organ able to regenerate itself, each girl would have a liver that would grow to a normal size after their separation.
"Each child will have a liver that can survive, produce everything it needs to produce and be able to drain all the important elements into the intestine to provide digestion," Stein said.
But the surgery still is dangerous and taxing.
"The overall surgery was about 7½ hours. Separating the liver, it took them about 3½," Jeff Roderick said.
Shawna and Janelle survived the risky procedure, but upon waking, the girls began to throw ferocious tantrums.
"Once they were separated, they actually just would throw a fit because they were used to somebody right there in their face," Michelle Roderick said.
The girls recovered from the trauma over separation and over time. Today the girls are 11 years old and thriving.
"I don't think it's very apparent that they were conjoined from the outward appearance," Michelle Roderick said. "There are a few scars that only are visible if you look for them."
The girls said they couldn't even fathom being constantly connected physically.
"If we were still stuck together, that would be kind of weird," Shawna said. "You wouldn't be able to do a lot of things."
"We're happy that we're separated. I am at least," she added.
Different Types of Conjoined Twins
Thoracopagus twins, like Shawna and Janelle, are just one type of conjoined twins. Other types are more difficult to separate.
For instance, ischiopagus twins, who connect at the torso, can share a pelvis, liver, intestines and reproductive systems, while cephalopagus twins have conjoined brains, necks and chests.
Finally, there are pygopagus twins, which connect at the pelvis, spine and bowels.
Even though the source of conjoined twins is a medical mystery, science knows how identical twins occur.
When a fertilized egg splits into two distinct individuals, the result is identical twins.
One theory suggests conjoined twins occur through a process called fusion, where the twins begin fully separated.
"The fusion theory has a certain logic to say, perhaps the embryos were separate," said twins specialist Dr. John Templeton Jr., "and then for some reason they re-adhere to each other and then they might be joined."
Yet many doctors think fission, not fusion, causes conjoined twins.
This theory holds that a fertilized egg starts to split into two separate twins, but remains partly connected.
"Some of them may end up with an initial single umbilical cord, then divides into two," Templeton said. "And then somewhere around the 13th day, their separation is not complete."