Currently, there is no definitive answer as to why babies yawn while in the mother's womb. Although all vertebrates, including fish and birds, exhibit the act of yawning, in humans this phenomenon begins around the 11th week of gestation, making it extremely difficult to study because it can only be detected through ultrasound.
In practice, analyzing fetal yawns requires prolonged and repeated monitoring of the fetuses, in addition to considering variables such as gestational age and different maternal characteristics. Furthermore, scientists themselves disagree on the exact definition of what constitutes a yawn, since babies make various movements in the womb, raising doubts whether the movement is a yawn or just a casual oral stretch.
Even outside the uterine environment, the function of yawning still generates much discussion. The theory that yawning aims to oxygenate the brain, although widely accepted, faces barriers to proof; for example, even in environments with oxygen-enriched air, people continue to yawn in response to other stimuli.
Besides drowsiness, several factors can influence the occurrence of yawns, such as stress, hunger, pain, excitement, the need for body temperature regulation, and the presence of another person yawning.
Fetal yawning remains a blind spot for many of these explanations. It does not seem logical that an intrauterine yawn could, for instance, indicate sleep or aid in thermal regulation. However, more recent research has provided relevant data for this area.
In February 2026, a study published in the scientific journal Plos examined the yawns of 32 healthy fetuses. The researchers adopted a classification that considers elements beyond the simple opening of the mouth, such as the stretching of other limbs and the speed of mouth opening and closing.
With this methodology, it was found that throughout gestation, the fetuses exhibit the same frequency of yawning observed in newborns and babies, varying between two and five times per hour, on average. The investigation also revealed that low-weight fetuses tend to yawn more, leading the authors to suggest the possibility that fetal yawning may be linked to some type of stress control, given that low fetal weight is correlated with other indicators of stress.
The researchers emphasize, however, that they still do not know all the intrauterine mechanisms involved and that determining a precise cause is very complex. They also point out that fetal yawning may not have an evolutionary function, being merely a byproduct of a mechanism that only manifests completely after birth.
Subsequently, in May of the same year, another study, published in the journal Current Biology, presented an even more intriguing result: yawning may be contagious both between the mother and the fetus. Pregnant women participated in a test by watching three types of videos: one with people yawning, another with people opening and closing their mouths, and a third with people standing still.
During the screening, cameras recorded their faces, and an ultrasound device monitored the facial movements of the fetuses in real time. Upon seeing yawns, approximately 64% of the mothers yawned, and slightly more than half of the fetuses reacted similarly about 90 seconds later, an interval comparable to that observed in contagious yawning between adults. This effect did not occur in the other phases of the experiment, when videos of people only moving their mouths or maintaining a neutral expression were shown.
The researchers also noted that mothers who yawned more frequently tended to have fetuses that showed this behavior with greater incidence. Although this does not yet constitute proof of the reason for fetal yawns nor a cause-and-effect relationship, it represents an association that may help scientists unravel this mystery.