I agree with fourlittlebirds--I think our bodies shake for different reasons.
I have never given birth, but I can almost guarantee that I will shake after I do. I'm just a shaker--I shake when I'm overly upset or when I'm overly happy or when I'm overly nervous. Anytime I feel my face flush and my adrenaline rush, I know I'm about to start shaking. And they're not just little shivers either, they're whole body shakes.

On the other hand, I worked as a veterinary technician for a few years, and the surgery animals always shook while waking up from anesthesia.
So, if I had to guess, I'd say women shake after birth because of either an emotional (adrenaline) or physical (change in temperature/low blood sugar) body response. I'm just guessing here, though.

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