It’s an uplifting news story: one of two conjoined twins is headed home after 13 months of recovery from multiple surgeries, and his brother is recovering fine as well. But the reality of conjoined twins is a sad one, and the question is if science will ever be able to achieve a high survival rate for such a situation.

Conner Mirabal has been cleared to leave Wolfson Children’s Hospital in Florida, and his brother Carter has remained in recovery after the procedure in December 2014 and multiple surgeries since then, as we reported recently. The boys were conjoined from the sternum to the lower abdomen and had to go through a series of complicated and delicate surgeries that culminated in them finally becoming separated May 7.

But most conjoined twins aren’t quite so lucky.

Conjoined twins are fused together and typically share skin and internal organs — organs that are necessary for survival. Conjoined twins are rare, occuring once in every 200,000 live births. A full 40 to 60 percent of them arrive stillborn, and 35 percent survive just a day. The overall survival rate is abysmal: conjoined twins live 5 to 25 percent of the time.

Female siblings for an unknown reason tend to survive more often than male conjoined twins. Female twins are three times more likely to be born alive than males, and 70 percent of conjoined twins are girls.

Conjoined twins develop from the same fertilized egg. Typically, this would result in identical or paternal twins, but in the case of conjoined twins, the woman’s egg doesn’t fully separate after being fertilized, and the embryo starts developing into identical twins before suddenly stopping a few weeks after conception. This results in a conjoined fetus.

This illustrates why it is so difficult for doctors to separate them safely: they often have shared organs, and separating them becomes an extremely risky procedure that science hasn’t quite nailed down. Oftentimes, doctors won’t even attempt it.

The University of Maryland has more information on conjoined twins here.