Ten years after a dangerous operation that separated conjoined twins Carl and Clarence Aguirre, their mother thanked the New York doctors who made the surgery a success. It is, after all, extremely rare to see both conjoined twins survive birth and an operation.

At the 10th anniversary celebration at Montefiore Medical Center, located in the Bronx, Arlene Aguirre expressed her gratitude to Dr. James Goodrich and his team, saying she had put all her hope and trust in the surgeons. Now, both of her sons are alive and well. The family is originally from the Philippines but lives in the U.S. on a medical visa.

In 2004, Goodrich and his team separated the twins, who are now 12 years old. Though Clarence is now in near-perfect condition, his brother Carl still has some difficulties when it comes to talking and walking. “When they were born, the doctors at home [in the Philippines] told me, ‘You have to choose which one is to live,’” Arlene Aguirre told CBS News. “I said, ‘I cannot choose that.’ The doctors here did not ask me to choose.” The boys were born joined at the head, unable to sit up, stand, eat, or see each other. Once they were separated, Clarence began improving more than Carl — but at least they both survived.

Conjoined twins are a rare phenomenon that occurs once in every 200,000 live births. Forty to 60 percent of these babies are stillborn, and 35 percent of those who are born survive only one day. Separating them is extremely risky, and survival is minimal — and having both of the twins survive is even less likely. Despite the high chance of mortality during surgery, however, many conjoined twins who are now separated have gone on to live full and healthy lives. Emily and Caitlin Copeland, for example, were separated at one years old and are now 18, graduating high school as co-valedictorians, and going off to college.

But many conjoined twins are born with risky situations, such as having joined hearts, which makes it nearly impossible to operate. Some grow up joined and are able to live without too many complications. The first record of conjoined twins is of Mary and Eliza Chulkhurst, or the Biddenden Maids, who were born sometime in the year 1100, according to legend. Some historians don’t believe the twins existed, while others claim they did in fact live, well into their 30s. Other famous conjoined twins include Chang and Eng Bunker, who were born in Siam in 1811; Donnie and Ronnie Galyon, and Millie and Christine McCoy.