Around 1 in 250 pregnancies result in identical or “monozygotic” twins, but the mechanism behind monozygotic twinning has remained elusive, partly because the phenomenon is so rare and partly due to ...

A simulation reveals how conjoined twins Abby and Brittany Hensel's body functions after siblings spotted with newborn baby.

Mallory Carlson tells PEOPLE she's doing everything she can to help her daughters Josie and Lucy survive leukemia ...