How early can braxton hicks occure?

Helen
I am dehydrated, and i know that that can cause braxton hicks. I am 16 weeks today, and can feel my uterus tightening many times a day. 
At first i thought it was the twins moving to the top of my uterus, but now I am not so sure anymore. 
I really hope that this doesen't affect my cervix...  

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