Thank you for asking this interesting question Hazel, I am sure many patients are keen to know as well.
If you remembered some chemistry from your school days, iron oxidises, or rusts. So the longer the iron or the lining has been in your body, the more likely it will "rust" inside.
It is absolutely normal to have brownish or even black colour menses on the first few days. But of course if you are worried, you can see a GP or Gynae to have it checked.
Take care!
