HCG, MIU Blood level

Is it normal for my first beta to come back at 18? I had my blood drawn yesterday to confirm pregnancy. My doctor mentioned me going back in Monday and Wednesday for a more accurate 48hour reading. I’m trying not to worry or stress myself out but I can’t stop thinking about it. I recently had a miscarriage in august. I’ve read that during week 3 it should be 5-50 but then I’ve also read that if it’s lower than 24 is concerning? I have continued taking tests and they do seem to be getting darker, I think that’s how I’m keeping peace of mind right now.