I had this thro both my pregnancies, It did my head in, I eventually found out that its loose pelvic muscles, its actually the muscle vibrating. Try doing pelvic floors, and do it when you get the vibrating too. Muscles loosen up around there alot during pregnancy.
BTW - sorry for gatecrashing 2nd tri....I came across this on new threads & knew I knew the answer loooooool
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