Due date question

So based on my first cycle day, both Glow and my GP calculated my due date as Sept 13 which makes me 6 weeks 4 days today. I did an ultrasound yesterday and panicked that baby is too small. However, what I forgot to mention to my doctor is that I ovulated very late last month, on CD 24. I looked online and it seems that it makes a difference and makes me 5 weeks 3 days. Does anyone know anything about this? I'm scheduled for another U/S in 10 days but if I'm inly 5 weeks now I want to postpone U/S to my 7th week.