The issue is if a person doesn’t seek medical attentions (as is the case with a lot of chemical pregnancies) it will be impossible. All those who attend hospital and are confirmed as having a miscarriage are counted. I personally didn’t attend the GP or hospital when I went through an early miscarriage as being a midwife I knew the management would just be monitor bleeding. Attend if really heavy and test again 3 weeks later. This is the case for many. Especially those who have been through multiple miscarriages so it’s an impossible thing to count
I agree it could never be completely accurate but getting a rough figure could still help.
What do they consider a miscarriage that is required to be recorded? I live in Australia, so I don't know what the rules/laws/expectations are in the UK. I was just curious if it's a matter of the gestational age or something else?