Babies Who Yawn More Often in the Womb Tend to Be Born With Lower Birth Weight

Yawning is one of the earliest behaviours exhibited by humans — even before birth. Now, a recent study reveals a striking finding: fetuses that yawn more frequently in late pregnancy tend to be born with a lower birth weight.

The finding suggests that simple facial movements such as yawning may carry important clues about fetal growth and condition long before labour.

The research was published in the scientific journal PLOS ONE.

During routine ultrasound exams, researchers observed a fetus opening its mouth slowly, holding it for a moment, and then closing it again — a pattern identified as yawning.

Damiano Menin of the University of Ferrara analysed the recordings to quantify how often each fetus yawed. He then linked this frequency to the baby’s birth weight.

The results were consistent: among healthy term babies, those who yawned more in utero tended to be born lighter.

Although all babies in the study were healthy and born at term, the pattern remained clear. It opens the possibility that yawning frequency could be a subtle indicator of fetal growth processes.

To ensure accuracy, the team used a strict definition. True yawning has three stages: the mouth opens slowly, a brief pause, then the mouth closes smoothly.

Two trained observers reviewed the videos frame by frame to separate this movement from faster mouth openings and other facial movements that are not yawns.

The method was applied to 32 fetuses aged 23–31 weeks of gestation — a period when fetal facial movements are particularly evident.

Each scanning session lasted about 22.5 minutes, and only moments when the fetus’s face was clearly visible were counted.

The results: yawning is relatively infrequent compared with other fetal facial expressions.