6+1, fetal pole measuring 4mm, no heartbeat

Danielle
I'm kind of panicking and you guys are all I have as I'm so early on. 
Last week I went for my first ultrasound, based on LMP I should've been 6w5d but I measured 5w1d and all that was detected was a yolk sac. My cycles are crazy so I figured that's why I was measuring small. 
Went back this morning for a second ultra and I was measuring 6w1d, they DID find a fetal pole this time, and everything looks okay except there was no heartbeat detected. 
The doctor said that the pole was measuring 4mm and typically they see a heartbeat when it's 5mm long. She said not to get discouraged yet.
I go back next week for another ultra. But I'm worried. This is my 3rd pregnancy, I miscarried in 2012, had a son in 2013 and now this pregnancy. 
Any advice? Anyone have no heartbeat at 6+1 and carry to term?