How accurate are ultrasound measurements..

I need as much hope as I can get because all I’ve been getting is stories were things did not turn out good.

-August 11th BFP (irregular periods)

-August 23rd first ultrasound since we don’t know exactly how far along I was. I did have a period July 10th but they are irregular. We guessed 6 weeks but ultrasound measured between 4-5weeks. I was worried about an empty sack...

- September 9th(2 weeks after 1st ultrasound) baby had grown! Measuring around 6w3d and 6w4d. I was supposed to be around 7w4d. We could not find a heartbeat. Midwife thought she saw one but lost it then doctor came in and couldn’t find it.

Midwife said “could be a missed miscarriage”

Doc said “baby is to small to see heartbeat or in a bad position, come back in a week”.

So now I’m freaking the f**k out! If I was 6w4d then I would have been 4w1day at first ultrasound which DOES measure out correct because they did say I was between 4-5 weeks at my first one. BUT I did get my BFP August 11th... that’s 2 weeks before they said i was 4ish weeks.. so does this all add up at all?? I’m trying to hold on to as much hope as possible. I keep having bad nightmares. I never imagined this happening to me.