MT recently reported on a Minnesota woman who had delivered conjoined girls, knowing that one would have to die — but why is it that it is so difficult for conjoined twins to survive once they are born?

Amber McCullough delivered Hannah and Olivia on Wednesday in Colorado, and there was a five-hour surgery to separate them. Olivia did not survive, but Hannah is now in critical but stable condition following the surgery. McCullough said that she was “believ[ing] in the power of prayer and the talent of the medical professionals, she said according to an ABC News report.

Olivia was not expected to survive the operation and died, but Hannah is in critical but stable condition. McCullough said she “believe in the power of prayer and the talent of the medical professionals,” she said according to the report.

Conjoined twins are essentially identical twins that have been fused in utero, a rare phenomenon the frequency of which is disputed, but estimates range between 1 in about 50,000 births and 1 in nearly 200,000, with the highest concentrations in Southeast Asia, Brazil, and Africa.

Of these births, many of them don’t survive, with about half ending up stillborn. And then there are cases like McCullough’s, where one survives but the other dies. A smaller fraction of the pairs actually survives after being separated, and yet a smaller fraction will survived being fused together and go on to live into adulthood.

However, these pairs often have abnormalities that makes life difficult. The problem with conjoined twins is they often share things like vital organs in a way that makes it so two bodies cannot survive on them. Separation is usually the goal, but sometimes they are so thoroughly fused together that it’s impossible.

It depends on the situation. Sometimes, separating conjoined twins is quite easy, but other times it’s nearly impossible and possibly life-threatening. Twins that are joined at the head or that share a vital organ are least likely to survive.

The issue of conjoined twins is actually quite near and dear to the heart of a current presidential candidate. Ben Carson, a GOP candidate, made medical history back in 1987 by being the first surgeon to successfully separate conjoined twins who were joined at the back of the head. It took Carson and his team 22 hours to successfully separate the two.