Braxton hicks...

Just • 2 girls & baby girl no3 on her way 😘
How early can Brixton hicks starts? 
I'm 20 weeks on Saturday and for the past week been having headrin hicks. Should I be worried that it's so early? This is my second baby btw and never had them with my first and if I did I defiantly didn't notice them or know I was having them. 
Thanks in advance