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Do Facial Expressions Develop before Birth?
Figure 1
Illustration of developmental progression in number of action units (AUs) over gestational age.
(a) One AU at 24 weeks (b) Two AUs at 27.5 weeks (c) and (d) two different combinations of 4 AUs at 32 weeks.
doi: https://doi.org/10.1371/journal.pone.0024081.g001