Late fertilization

I only BD twice last month. 2/12 and 2/14. I got my first positive 2/28. Based on “conception” or so I thought I should be 6w6d today. I had an appointment yesterday, saw the beginning of pregnancy and we could see a heartbeat but could not pick it up. She said I think that’s the baby there and measured me at 5w5days. So today I am 5w6d, a week earlier. So that means it took me longer to fertilize... should I be concerned?

Note: My HCG levels were 44K on Saturday about 48 hours later I was at 58K.

I guess I didn’t know it could take that long for everything to happen.

I’ve had two miscarriages so I’m just curious.