Guaranteed heartbeat at 6 weeks?

Kimberlee • Mama x3 | 30 | Married ❤️
I don't go in for another sonogram until December 17th, & I'll be 9 weeks. My husband and I weren't planning on announcing it on Facebook till we hear the heartbeat, but we're really want to post it on Thanksgiving. We think it's a perfect time to do it. What I'm wanting to know is if there is a guaranteed heartbeat at 6 weeks? Thank you in advace! ♡♡♡♡