"For the study, published on Wednesday in PLoS One, scientists scanned 15 healthy fetuses, eight girls and seven boys, at 24, 28, 32 and 36 weeks' gestation. They distinguished yawns from jaw openings by the timing of the action and shape of the fetuses' mouths. In all, they counted 56 yawns and 27 non-yawn mouth openings. By 36 weeks, the yawning had completely disappeared.
Why fetuses yawn is unclear — for that matter, it is unclear why adults yawn. In any case, the study's lead author, Nadja Reissland, a developmental psychologist at Durham University in England, said that yawning in a fetus is different from yawning in adults.
"When you see a fetus yawning, it's not because it's tired," she said. "The yawning itself might have some kind of function in healthy development. Fetuses yawn, and then as they develop they stop yawning. There's something special in yawning."
Image: Baby yawning, via Shutterstock