Although this unsuspecting husband does not initially know the contents of the bag or its ramifications - he believes something is up a bit earlier in this video (0:34 - 0:38). A moment later (captured below during 0:51) he knows he's going to be a father. This requires some clarification though - for his expression is classic on of classic emotional shock.
Emotional shock can be thought of as a more dramatic form of surprise, however some emotional experts (including myself) disagree and think of psychological (emotional) shock is significant of a (temporary) disconnect between what the emotional brain is still trying to grapple with - versus what the intellectual brain already knows. Of course, depending on the stimulus, the degree of emotional shock can be quite variable.
Like surprise, his eyes (eyelids) are opened significantly wider than baseline (normal). Also similar to surprise, his mouth is opened. It's crucial that with both emotions (surprise and shock) there is almost never any teeth visible. With classic surprise however, the mouth is opened much wider - in a vertical oval shape. Opening the mouth widely in such a fashion requires active muscle contraction (e.g., Nonverbal Communication Analysis No. 3106: Amazing Selfie Proposal - Body Language Components of Surprise). It's an important physiological distinction then - that with psychological shock - the jaw drops open (with no muscle contraction) - thus the colloquial, "Slack Jaw" or "Slack Jawed".
After a few seconds (sometimes longer) later, when the intellectual mind and the emotional mind become congruent, this emotional shock is replaced by other emotions (just as with surprise) - here it is replaced by joy-happiness (and of course it may, in other situations anger, fear, sadness, etc. may take its place instead).
Congratulations sir. Cherish every second.
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