24-week old fetus yawning

24-week old fetus yawning

Credit: Nadja Reissland

Though the exact purpose of yawning is a mystery, 4-D ultrasound images reveal that fetuses yawn in utero

27-week old fetus

27-week old fetus

Credit: Nadja Reissland

Yawns can be distinguished from feeding reflexes because yawning mouths spend longer open and have a shorter closing period.

28-week old fetus

28-week old fetus

Credit: Nadja Reissland

As babies mature, they yawn less frequently. Researchers believe yawning may help trigger brain development.

32-week old fetus

32-week old fetus

Credit: Nadja Reissland

Because yawning frequency changes throughout pregnancy, yawn patterns could be used as a marker of healthy development.

Yawning in utero

Yawning in utero

Credit: Wolfgang Moroder

By the time babies are full-term, they have stopped yawning in the womb.

Fetal Faces

Fetal Faces

Credit: Nadja Reissland

Researchers used ultrasound images of fetuses' faces, like these, to track how they used progressively more complex facial movements.