Fetal Yawning Linked to Lower Birth Weight: New Study

Babies may initiate displaying some of their most fundamental behaviors—even before birth. A new study reveals a potential link between fetal movements and future health, finding that babies who yawn more frequently in the womb are more likely to be born with a lower birth weight.

The research, published in the journal PLOS One, suggests that even simple facial movements like yawning could offer important clues about a baby’s growth and condition well before delivery. Understanding these early indicators could help healthcare providers better assess fetal development.

Researchers at the University of Ferrara analyzed ultrasound recordings to observe fetal yawning—defined as a sluggish mouth opening, a brief pause, and a gentle closing. Damiano Menin led the analysis, carefully counting how often each fetus yawned and correlating it with the baby’s weight at birth.

The results consistently showed that among healthy, full-term babies, those who yawned more often during gestation tended to be born lighter. This pattern remained noticeable even within the study’s healthy population, suggesting that yawning frequency might be a subtle indicator of the developmental process.

Distinguishing between a true yawn and other mouth movements can be challenging in ultrasound images. To ensure accuracy, the research team used a strict definition: a genuine yawn involves a slow, deliberate opening of the mouth, a short pause, and a smooth closure. Two trained observers reviewed the videos frame-by-frame to differentiate yawning from quicker, non-yawn mouth movements.

The method was applied to 32 fetuses between 23 and 31 weeks of gestation—a period when fetal facial movements are clearly visible. Each scanning session lasted approximately 22.5 minutes, with only periods when the fetal face was clearly visible being counted.

The study found:

• Yawning frequency ranged from 0 to 6 times per session.
• The average rate was 3.63 yawns per hour of effective observation time.
• Non-yawn mouth movements occurred much more frequently.

This indicates that yawning is a relatively rare movement compared to other fetal facial expressions.

A low birth weight is defined as less than 2.5 kilograms, and babies born at this weight are at increased risk for certain health problems. The findings could potentially lead to new ways to monitor fetal well-being and identify babies who may benefit from additional support after birth.