Fetal yawns are rare and tied to birth weight, not gestational age
Mar 7th 2026
In a 4D ultrasound study of 32 healthy fetuses, researchers found yawns to be uncommon and stable across 23 to 31 weeks, while non-yawn mouth openings declined with age, and higher fetal yawning rates were linked to lower birth weight within the normal range.
- Study recorded 32 healthy fetuses between 23 and 31 weeks using 4D ultrasound and Baby FACS/SCPB coding.
- Median yawning frequency was about two yawns per hour and the mean was 3.63 yawns per hour, well below 5 per hour.
- Yawning frequency showed no significant relationship with gestational age across the sampled window.
- Non-yawning mouth openings decreased in frequency with gestational age and their average duration also declined with age.
- Yawning frequency was negatively associated with birth weight, with higher yawning linked to lower birth weight within the normal range.
- Inter-rater reliability was high with Cohen’s Kappa of 0.88 for mouth opening identification and 1.00 for classification as yawns or non-yawns.
- Limitations include a small sample, short observation times, exclusion of high-risk pregnancies, and lack of direct maternal or fetal physiological measures such as temperature or stress biomarkers.