A somersault was a type of gymnastics move.
The movements of an unborn baby in the womb were sometimes compared to somersaults. Keiko O'Brien assured William T. Riker that her baby was just doing somersaults in 2368 when she invited him to feel her move. (TNG: "Disaster")
In 2372, Ekoria wasn't surprised that her baby's head had moved into a new position, saying it felt like he was turning somersaults inside her. (DS9: "The Quickening")