Researchers at Durham University said the process was "adaptive" and helped the unborn baby to prepare for life after birth.
The study expands on previous research that suggests that facial expressions of healthy foetuses develop and become more complex during pregnancy.
"It is vital for infants to be able to show pain as soon as they are born so that they can communicate any distress or pain they might feel to their carers and our results show that healthy foetuses 'learn' to combine the necessary facial movements before they are born.
"This suggests that we can determine the normal development of facial movements and potentially identify abnormal development too.
"This could then provide a further medical indication of the health of the unborn baby."
More top news
The ideal man or the best the Black Cats could do? Martyn McFadden blogs on Sunderland's new manager.
Scotland are into the World Cup quarter-finals, but only after a nerve-shredding 36-33 win over Samoa in Newcastle.
Emma brings you Saturday night's forecast for the Tyne Tees region