Do you breathe in while taking your measurements?

I'm trying to take my bust measurement but I'm really struggling to get a good number. Because if I breath I have one number. But when I breathe out the measuring tape gets loose. If I measure when I breathe out and the breath in the tape cuts into my skin. The difference between the numbers is also several centimeters. So what would be the right number because I don't know and I've struggled with this, always afraid to ask😅

Edit: so first of all thank you all for your comments that was really helpful and nice of you! And I tried taking my measurements again while breathing with my stomach (don't know how to call it in english) and also while taking small breaths. I ended up with the bigger number again both times so I think that's the correct number and I will go with that. Thanks again I appreciate everyone's help and detailed comments!