Ultrasound scans of faces in utero can distinguish between when a fetus yawns and when it just opens its mouth, according to research published November 28 in the open access journal PLOS ONE by Nadja Reissland from the University of Durham and colleagues at other institutions.
The researchers found that during the period between 24 to 36 weeks of gestation, they could distinguish between yawns and simple mouth openings on ultrasound images of fetal faces. They report that simple mouth openings occur less often than yawns and the frequency of these openings declines from 24 weeks of gestation onward.
More than half of all mouth openings are yawns at 24 weeks of gestation, and the frequency of yawns begins to decline at a later stage (28 weeks onward).
The study suggests that trends in yawning behavior may potentially be used to track healthy fetal development. Reissland says, "Fetus yawning is not contagious, nor do they yawn because they are "sleepy". Instead, frequency of yawning in the womb might be related to activity-dependent brain maturation early in gestation."