(CNN) -- The chances that conjoined twins will survive are minimal. The chances that both conjoined twins will survive surgical separation are even tinier. That's why a Pennsylvania couple says they will keep their two newborns together.
"I'm thankful they were able to survive this long, and they're still going strong," their 25-year-old father, Kody Stancombe, told CNN affiliate WTAE.
"It would hurt us to lose one and have the other," the twins' mother, Michelle Van Horne, said. "They were born together; they can stay together."
The couple from Indiana, Pennsylvania, gave birth to the twins on April 10. Andrew Donovan Lee and Garrett Lee Donovan Stancombe are connected from the breastbone to the waist. They share a heart and a liver. The medical term for that is omphalopagus.
Some 33% of conjoined twins are connected in this manner. In most of these cases, the twins rarely share a heart.
Sharing a heart makes a separation surgery particularly risky. There are no known cases where twins with hearts joined at the pumping chamber have survived that surgery, according to the University of Maryland Medical Center.
Doctors have gotten better at separating children who share other organs.
In 2006, surgeons successfully separated Utah twins Kendra and Maliyah Herrin. The 4-year-olds were attached at the abdomen and shared a pelvis and kidney. It took doctors 16 hours to separate them, but the operation went smoothly.
Conjoined twins are extremely rare. They occur once in about every 200,000 live births. Conjoined twins happen when a woman produces a single egg that does not separate completely after it is fertilized. If it did divide completely, then the woman would give birth to identical twins. Fraternal twins are born from two separate fertilized eggs.
The overall survival rate of conjoined twins is from 5% to 25%, with about 75% of surgical separations resulting in at least one twin surviving. Approximately 200 pairs of conjoined twins are born alive each year, and about half die before their first birthday.
The first known case of surviving conjoined twins happened in England in 1100. Mary and Eliza Chulkhurst were joined at the hip and lived for 34 years.
Conjoined twins were at one time called "Siamese twins," named for two brothers, Eng and Chang Bunker, who came from Siam, the country now known as Thailand.
After appearing in circuses and sideshows all around the world, they settled in the United States and become successful ranchers. They had 21 children between them after marrying two sisters. The term "Siamese twins" is no longer used since it is considered racially insensitive. The Bunker brothers lived until 63.
Andrew Donovan Lee and Garrett Lee Donovan Stancombe seem to be doing well. They are breathing on their own and are doing all the things healthy babies do -- sleeping, crying and eating.
"For me, (the best part) is being able to hold them and hear them cry and know they're here with me," Van Horne told CNN's affiliate WTAE. "Definitely, changing their diapers and bathing them is a two person job for me."