Fetuses start yawning in the womb during the first trimester.

Research published in the journal PLOS ONE in 2012 utilized 4D ultrasound technology to distinguish fetal yawning from simple mouth opening. Led by Dr. Nadja Reissland of Durham University, the study monitored 15 healthy fetuses and found that yawning occurs most frequently at 24 weeks of gestation. While the frequency of yawning declines after 28 weeks, the initial onset begins as early as the 11th week. Unlike adults, who yawn due to tiredness or boredom, fetal yawning is linked to brain development. Scientists believe the repetitive motion of yawning triggers specific neural pathways necessary for the maturation of the central nervous system. This activity is also essential for the physical development of the temporomandibular joints and the muscles required for nursing and vocalization. The study also noted that there was no significant difference in yawning frequency between male and female fetuses. By observing these patterns, medical professionals can potentially use yawning frequency as a biomarker for healthy fetal development. This discovery highlights that even the most basic human behaviors serve a critical biological purpose long before a child enters the world.