Observational studies of 4D fetal scans at Durham and Lancaster Universities show that fetuses are capable of yawning, demonstrating a developmental process.
Nadja Reissland, a researcher at the University of Durham in the United Kingdom, applied a more detailed ultrasound technique to the study in order to get images of fetal faces that could distinguish an actual yawn from just an open mouth, as described in a NPR article.
In the studies, Science Daily reports that observations were carried out on eight female and seven male fetuses from 24 to 36 weeks gestation. The researchers found that yawning declined from 28 weeks and that there was no significant difference between boys and girls in yawning frequency. By the time babies are full-term in the womb, they have stopped yawning.
The studies are showing that fetus yawning may be a developmental process, giving doctors tools to use for a diagnosis of the festus' health when born. According to Reissland, "They seem to open their mouths widely much less often than they yawn." She also found that yawning was common at 24 weeks but then dropped to zero at 36 weeks, according to a prestigious international academic report, published by Reissland and her colleagues in the journal PLOS ONE.
The study is showing that yawning by adults and by fetuses are different in nature, with children under five immune from the adult's "contagious nature." One person's yawning causes those around to also begin yawn.
Developing fetuses are thought to trigger brain maturation through self-stimulation, or yawning. However, Nadja Reissland says one of their studies showed that when born, babies do not imitation the yawns of their mother, but will imitate mouth movements like pursed lips and smiles. "How is it possible that these babies can imitate mouth movements, but they don't imitate the yawning?" Reissland wonders.
"It could be that yawning is something which you need in order to have a functioning brain, which is a hypothesis," she says, adding that she would like to compare yawning in healthy fetuses, like the ones she studied, with yawning in fetuses that have medical conditions.