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The 4-D scans of 15 healthy fetuses by Durham and Lancaster Universities has put an end to the debate among researchers about fetuses being able to open their mouth inside the womb.

The study findings, where ultrasound scans have shown fetuses yawning in the womb, also suggest that yawning is a developmental process, and this could potentially give doctors another index of a fetus' health.

The findings distinguish 'yawning' from 'non-yawn mouth opening' based on the duration of mouth opening with the help of 4-D video footage which closely examines all events where a mouth stretch occurred in the fetus, Medical Xpress reported.

In this study, researchers could determine that more than half of the mouth openings observed in the study could be categorized as yawns.

The study, which was carried out on eight female and seven male fetuses aged 24 to 36 weeks, also determined that the frequency of yawning reduced from 28 weeks and that the gender of the fetus was of no significance in yawning frequency.

"The results of this study demonstrate that yawning can be observed in healthy fetuses and extends previous work on fetal yawning. Our longitudinal study shows that yawning declines with increasing fetal age. Unlike us, fetuses do not yawn contagiously, nor do they yawn because they are sleepy. Instead, the frequency of yawning in the womb may be linked to the maturing of the brain early in gestation. Given that the frequency of yawning in our sample of healthy fetuses declined from 28 weeks to 36 weeks gestation, it seems to suggest that yawning and simple mouth opening have this maturational function early in gestation," lead researcher, Dr. Nadja Reissland, of Durham University's Department of Psychology, said.  

She also added that yawning may be a part of the development of the central nervous system in fetuses, but then further studies are required to examine this theory.

The study is published today (Nov. 21) in the prestigious international academic journal, PLoS ONE.