While it is well known that foetuses open and close their mouths, experts have disagreed over whether or not they are actually yawning.
A new study using high resolution ultrasound footage appears to confirm that they do yawn, and the most likely explanation is that it is linked to brain development.
The study of seven male and eight female foetuses from six to nine months of pregnancy was led by Dr Nadja Reissland of the University of Durham.
"The results of this study demonstrate that yawning can be observed in healthy foetuses and extends previous work on foetal yawning," she said.
"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 occur more frequently early in pregnancy and decline after 28 weeks.
The findings are reported in the online journal Public Library of Science One.
Both humans and animals yawn but exactly why is still a mystery.
Theories include communication, neurological function, regulating temperature and a link with stress.
The report said: "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, the report added.