When does something first show up on an ultrasound?

My first US was 4w+5d because of bleeding. Hcg was 52 the day before and then 101 that day then two days later was 212.

I then had my first follow-up at 5w+3d. Hcg that day was 460. Progesterone was low so I was put on it to supplement.

At both US, nothing appeared in my uterus. No sac, no baby, completely empty.

I have had on and off brown spotting the last few days and woke up with red today, although minimal in amount. No cramping.

My next follow-up is Thursday when I will be 6w+6d. Is it possible to see nothing at 5w+3d but then see the start of something at 6w+6d?? 10 days later?? I am so scared to go for a third US and still see nothing. Anybody else have multiple early scans like me?