How Soon Do We Start Yawning?

Yawing is a universal sign for sleepiness and boredom. Yawns are also contagious — observing a person yawn creates an empathetic chain reaction in others. In general, researchers believe that yawning is an arousal mechanism. Some theories suggest that yawns flush carbon dioxide out of our bodies, and others that yawning cools the brain. Some theories believe that our ancestors developed this behavior thousands of years ago, either as an intimidation tactic (bearing their teeth) or as a survival tactic (to help them stay alert while on the lookout for predators).  Therefore, it makes sense that yawns are hardwired into our brains at an early age. Seeing a baby yawn is pretty adorable. But who would have thought that babies who even aren’t even born yet yawn too. Why so soon?

That was the discovery I made last week as my husband and I were viewing a third ultrasound of our first baby. It’s been amazing to sneak peeks into our baby’s development, from when she was nine-weeks-old to five-months-old. In her first ultrasound, she looked like a little jumping bean. Now she’s a miniature version of a baby, squirming, tumbling, and waving her arms and legs about freely.

But I was stunned to watch her mouth open wide open for a few seconds, and then close again. The technician announced, “baby’s yawning.”

“How can that be?” I asked. “Babies aren’t breathing air in utero. Why do they need to yawn?”

“It’s probably boring in there,” replied my husband.

I was puzzled by this and began searching for an answer. It turns out that the earliest occurrence of yawning is 11 weeks after conception. There are plenty of theories about why we yawn. TLC’s How Stuff Works provides a summary of the physiological and evolutionary theories. But it still didn’t make sense to me why fetuses need to do this as a physiological behavior so early on.

Getting Sleepy

Fetuses don’t need to yawn for physiological reasons, but they’re most likely practicing it or forming primary neurological connections related to sleep and wake arousal. Fetuses practice a lot of behaviors before being born, some of which are important to survival. For example, they can swallow and smile. They can even hiccup, so it makes sense that they can also yawn.

But the fact that behavior of yawning begins so early puts it into a whole new light for me. I always thought embryology was a fascinating subject while studying in college. Our evolutionary heritage is wondrously revealed through a fetus’ development (for more information on this subject, I recommend NOVA’s episode “Life Greatest Miracle”). Yawning is a trait shared by many animals, and because this behavior makes its mark so early, it probably has an important link to our brain development and survival.

One researcher suggests that early yawning is an indicator of fetal neural development [1]. The brainstem that controls respiratory and basic sleep patterns (an immature form of REM sleep and wakefulness) forms around 6-8 weeks, before fetuses begin to yawn. Yawning and the development of REM sleep cycle occur simultaneously as the fetus’ brain develops. This could point to a possible link. While the process of awakening gets a little too complicated to explain here, yawning could be a reflex of the on/off sleep switch in the brain.

The bottom line is that yawning is still an incredible mystery to science. But it’s fun to find all the clues that point to its evolutionary link to our survival. In the meantime, I hope my baby enjoys learning her new behaviors. I’m looking forward to seeing her first yawn outside of the womb.

  1. Walusinski, O. [ed] “Fetal Yawning.” The Mystery of Yawning in Physiology and Disease. Front Neurol Neurosci. Basel, Krager, 2010, vol 28, pp. 32-41
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