Though the exact purpose of yawning is a mystery, 4-D ultrasound images reveal that fetuses yawn in utero
27-week old fetus
Yawns can be distinguished from feeding reflexes because yawning mouths spend longer open and have a shorter closing period.
28-week old fetus
As babies mature, they yawn less frequently. Researchers believe yawning may help trigger brain development.
32-week old fetus
Because yawning frequency changes throughout pregnancy, yawn patterns could be used as a marker of healthy development.
Yawning in utero
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.
Tia Ghose, Senior Writer
Tia has interned at Science News, Wired.com, and the Milwaukee Journal Sentinel and has written for the Center for Investigative Reporting, Scientific American, and ScienceNow. She has a master's degree in bioengineering from the University of Washington and a graduate certificate in science writing from the University of California Santa Cruz.