Waiting to enter the world is a tedious business - so perhaps it should come as no surprise that foetuses yawn.
Scientists have shown conclusively for the first time that unborn babies yawn repeatedly in the womb.
But it is not because they are sleepy or bored. The most likely explanation is that foetal yawning is an essential process linked to brain development.
While it is well known that foetuses open and close their mouths, experts have disagreed over whether or not they are actually yawning. The new study, using high resolution ultrasound footage, confirms that they really do yawn, and do it often.
Researchers used a standard definition of yawning to distinguish it from non-yawn mouth opening. A true yawn involves a slow opening of the mouth to its full stretch, followed by more rapid closure.
Dr Nadja Reissland, from the University of Durham, who led the study of seven male and eight female foetuses from six to nine months of pregnancy, said: "The results of this study demonstrate that yawning can be observed in healthy foetuses and extends previous work on foetal yawning. Unlike us, foetuses 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."
Foetal yawns occurred more frequently than simple mouth openings early in pregnancy, but declined after 28 weeks. In total, 56 yawns and 27 non-yawn mouth openings were observed from 58 scans. The findings are reported in the online journal Public Library of Science ONE.
Both humans and animals yawn but why this strange behaviour has evolved is still an unsolved mystery. Theories include communication, neurological function, regulating temperature and a link with stress.
The scientists wrote: "Given that the frequency of yawning in our sample of healthy foetuses declined from 24 to 36 weeks gestation, it is possible that yawning and simple mouth opening have a maturational function early in gestation. Although yawning and simple mouth opening have the same trajectory shape over age, it is notable that the yawning rate is just over double the non-yawning rate."
Further research was needed to examine whether foetal yawning is related to central nervous system development, they said.