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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.
Unborn babies "practise" facial expressions of pain while they are in the womb, scientists from Durham University say.
Foetuses have been pictured using 4D scanning technology showing what appears to be pain.
Researchers suggest the ability to grimace is a "developmental process" which could help doctors asess the health of the foetus.
The study, published in the journal "Plos One", found when the mother was 24 weeks pregnant, unborn babies were able to make simple expressions such as smiling.