SAN DIEGO, Calif. (FOX 5/KUSI) — How common are conjoined twins? And what causes this one body connection?

Last week, the oldest living conjoined twins, Lori and George Schappell, died in Pennsylvania at age 62. As reported by AP, through their lives they had pursued separate careers and relationships, all while defying medical expectations.

In another recent media buzz, former TLC reality stars Abby and Brittany Hensel, another set of conjoined twins, were brought back into the spotlight in March after it came out publicly that one of them got married. According to Today, the pair now work as fifth grade teachers.

This has some on the internet, specifically Reddit, wondering what causes this phenomenon.

The definition of a conjoined twin, according to the Mayo Clinic, is “two babies who are born physically connected to each other.”

Sometimes they may share organs or other body parts. They may be conjoined at the chest, abdomen, base or length of the spine, pelvis, head or chest.

Stanford Medicine states online that there are two theories about the development of conjoined twins in the womb, however, what might cause either chain of events to occur is unknown.

One of those theories is that a single fertilized egg, or early embryo, does not fully split. Put simply, the two babies that develop from this embryo remain physically connected.

Another theory is a fusion by two fertilized eggs occurs earlier in development, meaning two early embryos fuse together.

According to Stanford Medicine, the occurrence of conjoined twins is estimated to be between one in 30,000 to one in 200,000 worldwide, though the majority aren’t born alive.

“Because conjoined twins are so rare, and the cause isn’t clear, it’s unknown what might make some couples more likely to have conjoined twins,” stated the Mayo Clinic.