The Hensel girls are the rarest form of conjoined twins, the result of a single fertilised egg which failed to separate properly in the womb.
They have two spines (which join at the pelvis), two hearts, two oesophagi, two stomachs, three kidneys, two gall bladders, four lungs (two of which are joined), one liver, one ribcage, a shared circulatory system and partially shared nervous systems.
From the waist down, all organs, including the intestine, bladder and reproductive organs, are shared.
While they were born with three arms, one was removed surgically.
Although Brittany - the left twin - can’t feel anything on the right side of the body and Abigail - the right twin - can’t feel anything on her left, instinctively their limbs move as if co-ordinated by one person, even when typing e-mails on the computer.
It is rare for twins conjoined the way that Abby and Brittany are to survive into adulthood, but despite this they are in good health, without heart defects or organ failure.