On top of the survival rate of conjoined twins being extremely low, their mere occurrence is also rare: some estimate that one in 189,000 births brings a set of conjoined twins. Of that small population, only 11 percent are dicephalic parapagus twins such as Brittany and Abby.
The inner workings of their anatomy are just as remarkable as the twins’ outward appearance. Their upper body contains double of each organ – they have two stomachs, two hearts, four lungs, etc. – but their lower half is shared, with a single reproductive system, for example.
On top of that, Abby and Brittany each controls one half of their body, one arm and one leg each. As such, the girls had to work together to master crawling, clapping and walking. In their later years, they learned how to swim, run, brush their hair and even drive side by side.