Eighteen years ago, a set of twins were born conjoined at the hip with unknown survival rates, but today, June 10, they celebrate their birthday healthy and happy. Emily and Caitlin Copeland, from Houston Texas, defied the odds when they were successfully separated at 10 months old. They are set to graduate as co-valedictorians from Lutheran High North in their hometown of Houston, a reality that might have never been.  

"I think for anyone it's exciting to get to 18, but in particular for us I think it's just a really big blessing that we got to 18, considering what could have happened," Caitlin told the Associated Press.  

A set of conjoined twins are born out of every 200,000 live births in the United States. They are a set of twins that are born physically connected to each other, and in the case of the Copeland twins, they were connected at the liver, chest, and bile ducts. Most conjoined twin births are stillborn or they die shortly after birth, and those that survive can be surgically separated depending on where the twins are joined, what organs are shared, and the experience and skill required by a surgical team.  

"At the time, if you Googled conjoined twins all you got was circus acts and babies that died," the twin’s mother, Crystal Copeland told AP.  The statistics are intimidating for any new parent. An estimated 40 to 60 percent of conjoined twins are stillborn, and only 35 percent that are born alive will die after one day. The overall survival rate of conjoined twins is between five and 25 percent, depending on how they’re conjoined.  

"They were joined at the liver, not at the heart, which would have been, you know, fatal," Crystal said. "[The doctor] thought there were good opportunities for separation where they would both be able to live basically normal lives." 

Seventy percent of all conjoined twins are girls, an astoundingly high percentage with no identified reason for the gender disparity. They are always the same gender because they are genetically identical, which means they developed from the same fertilized egg.

In the case of conjoined twins, the egg doesn’t fully divide and the developing embryo begins to split into twins during the first few weeks of conception. However, the splitting stops and the twins remain physically connected in one way or another.  

"You don't always see the long-term results of what we do, and it's nice when you get to see a good one," said Dr. Kevin Lally, the twin’s doctor and surgeon in chief at Children’s Memorial Hermann Hospital in Houston.  

Out of the nearly a dozen different type of conjoined twins, Caitlin and Emily were omphalopagus twins. This common type occurs in about 33 percent of all cases, in which a liver, gastrointestinal, or genitourinary function is shared or connected. Although success rates have improved over the years, separation surgery is still a risky and delicate procedure. Since 1950, the survival rate for one twin is 75 percent, a daunting number.  

“We want to use this blessing to be a blessing to others,” Emily told ABC News. “I hope we can help so many people and just be, if anything, a story to read and smile [about] when everything looks bad.”