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."
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