Even in the womb, where all oxygen is provided by the parental placenta, fetuses can-and do-yawn. More yawns during observation were associated with a lower weight at birth-potentially indicating mild fetal stress in the womb, according to a study published February 25, 2026 in the open-access journal PLOS One by Damiano Menin, of the Università degli Studi di Ferrara in Italy, and colleagues.