Identical twins begin as a single fertilized egg that splits in two, and this installment of the In the Womb series uses 4-D ultrasound and computer graphics to follow that split through birth. Embryologists explain how twins sharing one placenta can end up with drastically different birth weights, one nourished at the other's expense, and trace rarer outcomes: a case where one twin's body absorbs the limbs of the other, and a case of ambiguous genitalia arising from early developmental crossover. The film also shows how skin color and other shared traits get written into both twins from that first cell division. A human story runs alongside the science: twins separated at birth who meet for the first time four decades later, their similarities in habits and choices offered as evidence of how much genetics shapes a life. Real-time 4-D footage catches twins interacting with each other inside the womb, pushing against the amniotic wall and against one another, months before anyone outside will meet them.