Though the exact purpose of yawning is a mystery, 4-D ultrasound images reveal that fetuses yawn in utero
Yawns can be distinguished from feeding reflexes because yawning mouths spend longer open and have a shorter closing period.
As babies mature, they yawn less frequently. Researchers believe yawning may help trigger brain development.
Because yawning frequency changes throughout pregnancy, yawn patterns could be used as a marker of healthy development.
By the time babies are full-term, they have stopped yawning in the womb.
Researchers used ultrasound images of fetuses' faces, like these, to track how they used progressively more complex facial movements.