Yes, babies yawn in the womb, and they start surprisingly early. Researchers using 4D ultrasound have captured fetal yawning as early as the first trimester, with detailed observations confirming the behavior is clearly present by 24 weeks of pregnancy. These aren’t just random mouth movements. Fetal yawns follow a distinct pattern that scientists can reliably distinguish from other mouth openings.
How Scientists Identify a Fetal Yawn
A baby opening its mouth in the womb isn’t automatically a yawn. Researchers have developed specific criteria to tell the difference. A true fetal yawn involves a slow, prolonged opening of the jaw to its widest point, followed by a quicker snap of closure. The baby’s head often tilts backward at the same time, and sometimes the arms rise slightly. In a simple mouth opening, by contrast, the timing is more symmetrical: the mouth opens and closes at roughly the same speed.
One widely used method defines a yawn as any mouth opening where the time it takes to reach maximum opening is longer than the time it takes to close back down. That slow-open, fast-close signature is what makes a yawn look like a yawn, even on grainy ultrasound footage. This distinction matters because earlier studies that didn’t separate true yawns from ordinary mouth movements likely overestimated how often fetuses yawn.
When Yawning Peaks and Declines
Fetal yawning doesn’t stay constant throughout pregnancy. A longitudinal study published in PLOS One followed 15 fetuses from 24 to 36 weeks and found a clear downward trend. At 24 weeks, fetuses averaged about 2 yawns per observation session. By 28 weeks, that dropped to around 1.4. At 32 weeks it fell to less than 1, and by 36 weeks, none of the fetuses were observed yawning at all.
More recent research has added nuance to this picture. A larger study estimated that the average yawning frequency across the second and third trimesters is below 5 yawns per hour, a rate similar to what’s seen in premature newborns. This study found no significant relationship between yawning frequency and gestational age, suggesting the steep decline reported in earlier work may have been partly due to non-yawn mouth openings being counted as yawns. Those ordinary mouth openings do clearly decrease as pregnancy progresses, which could have skewed the numbers.
What both lines of research agree on is that yawning is a real, identifiable behavior in the womb, and it’s more common in the second trimester than near the end of pregnancy.
Why Babies Yawn Before They Breathe
This is the part that puzzles researchers, because the most common explanations for yawning in adults don’t apply to a fetus. A baby in the womb isn’t tired, isn’t bored, and isn’t breathing air. So what purpose could yawning serve?
The leading theory ties fetal yawning to brain development. The fact that yawning is more frequent earlier in pregnancy, when the nervous system is undergoing rapid growth, and tapers off as the brain matures, suggests it may be linked to the developing central nervous system. The decline in yawning could reflect the brain reaching a stage where it no longer needs whatever stimulation or calibration the yawn provides.
Other researchers have proposed that yawning helps shape the jaw joint through repeated stretching of the muscles and connective tissue. There’s also a hypothesis that the wide-open mouth movement helps spread surfactant, the slippery substance that coats the inside of the lungs and is critical for breathing after birth. A more recent proposal suggests fetal yawning may aid airway development by repeatedly dilating the airway and repositioning the muscles around it, essentially rehearsing for what researchers describe as one of the most critical moments in life: the first breath.
None of these theories have been definitively proven. It’s possible yawning serves multiple developmental purposes, or that it’s simply a byproduct of the nervous system wiring itself together.
Does Yawning Indicate a Healthy Baby?
If you’ve spotted your baby yawning on an ultrasound, you might wonder whether it means anything about their health. At this point, fetal yawning is not used as a clinical marker. Doctors don’t count yawns during ultrasounds or interpret them as signs of a problem.
One interesting finding, though, is that yawning frequency appears to have a relationship with birth weight. The PLOS One study found that fetuses who yawned more frequently tended to have lower birth weights, while no such link existed with gestational age. The researchers noted this as a statistical association, not something that would be useful for predicting outcomes in an individual pregnancy.
The practical takeaway is straightforward: seeing your baby yawn on a scan is a normal part of fetal development. Not seeing it is equally normal, especially later in pregnancy. It’s a fascinating window into what your baby is doing in there, but it’s not something your doctor will use to assess how things are going.
What Else Babies Do in the Womb
Yawning is just one of dozens of movements fetuses practice before birth. By the second trimester, babies are also hiccupping, swallowing amniotic fluid, stretching, kicking, grasping, and making facial expressions that look like smiling or frowning. Many of these behaviors, like yawning, appear to be the nervous system testing out circuits that will be needed after birth. Swallowing helps develop the digestive system. Breathing-like movements (rhythmic contractions of the diaphragm without actual air exchange) prepare the respiratory muscles. Yawning fits into this broader pattern of the body rehearsing for life on the outside, long before the baby is ready to arrive.