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
Tyne and Wear Fire and Rescue Service is encouraging businesses to think about installing sprinklers, after a factory was almost destroyed.
Find out when the old pound coins will stop being accepted and the best way to deposit them so you don't lose out.
Music sensation KT Tunstall will lead a list of stars heading to the North East this summer for the South Tyneside Festival Summer Concerts.