Is it normal not to hear heartbeat and week 6 or 7?

I calculate I am on my week 8 but when I go for my 1st scanning it stated 5 week 6 days and its still a yolk sac. No heartbeat detected and its only 3.2mm. Doctor say come 2 weeks later and if still no heartbeat doctor say will have to accept no baby there. Is it true? This is my 3rd pregnancy and my other pregnancy was all good and now and I am 40 years old and it's 13 years apart from this new pregnancy. Need to feel better anyone wanna share experience with me. Thanks for replying..