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Ultrasound scans of faces in utero can distinguish between when a fetus yawns and when it just opens its mouth, says study published by PLOS ONE.

Kids in utero do the darnedest things. During ultrasounds, expectant parents delight in seeing an unborn child hiccuping, sucking a thumb – or cutest of all – yawning.

Until recently, however, many researchers believed that fetuses who looked like they were yawning were simply opening their mouths wide. But a new study published Wednesday in the journal PLoS One suggests fetal yawning can be reliably distinguished from a gaping mouth.

What's more, lead author Dr. Nadja Reissland, a psychologist from the University of Durham in the United Kingdom, says doctors may eventually be able to detect developmental abnormalities based on the frequency of fetal yawning.

"I think it's potentially a marker of healthy development," she says.

In the study, 15 healthy fetuses were scanned four times at 24, 28, 32 and 36 weeks gestation. A technician did 10-minute sessions of 4D ultrasound, which produces multidimensional images of the fetus in motion.

A yawn is generally defined as the mouth opening as the jaw drops. Reissland and colleagues differentiated between a yawn and a simple mouth opening by measuring the length of time it took to reach the fullest extent of the mouth stretch. In yawning, the opening part of the movement cycle was longer than the closing motion.

The researchers discovered that in both sexes, yawns were more than twice as frequent as simple mouth openings. But the number of yawns appeared to decline after 28 weeks gestation.

Reissland hypothesizes that yawning plays a role in brain maturation that is normally completed by 28 weeks. "Then [the fetus] moves on" to the next developmental phase, she says.

Previous studies of infants and children have shown a U-shaped developmental progression in yawning. Premature infants yawn more often than term babies and primary-school children yawn more frequently than kindergarteners.

Other researchers have observed that a baby won't mimic its mother when she yawns. Unlike adults, children up to age 5 appear to be immune to the contagious nature of yawning.

Why people yawn is a subject that has perplexed scientists for years. Some have suggested that yawning is a response to elevated levels of carbon dioxide or low levels of oxygen in the blood, but that theory has been debunked, Reissland and her colleagues note. Another theory, as yet unproven, is that yawning plays a role in cooling the brain.

Very few studies has been done on fetal yawning and the significance of the movement in fetuses is little understood, Reissland says.

The developmental function of yawning may be related to jaw movements necessary for building cartilage in the temporo-mandibular joint, enabling normal mouth movements, she and her co-authors write.

Another possibility is that fetal yawning is a sign of elevated blood levels of cortisol, a hormone associated with stress. To rule this out, future studies should measure maternal cortisol at the time of observing fetal yawns, she says.

Yawning is associated with sleepiness in adults but that relationship is unlikely in fetuses, whose arousal states are underdeveloped, Reissland points out. She speculates that fetal yawning, which occurs as early as the first trimester, may trigger specific brain functions.

"In fact, yawning might be a way of getting the brain to develop" so the neurons stimulated by the movement will then be able to trigger yawning after birth, she says.

Previous studies have found that anemic fetuses yawn more frequently than healthy ones. The same may be true of other compromised fetuses, Reissland says, adding that she hopes to study yawning in fetuses exposed to alcohol and drugs.

If researchers find significant differences, she says, excessive yawning "could be an early indicator of unhealthy development."