Early miscarriages (5 weeks) are very common and most women will never even realise they had one!
A chemical miscarriage is when you do the test a week early (using sensitive test like First Response) and then a week later you get your period.
If I didn't test early every month I may never have know that I got pregnant.
A very common cause of early miscarriages is APS:
www.tommys.org/pregnancy-information/pregnancy-complications/antiphospholipid-syndrome-aps
Its the cause of 25% (?) of miscarriages and most women will never know they have it unless you ask GP to test for it (like I did).
If you have APS you simply take low dose aspirin/heparin to stay pregnant.
On Mumsnet everyone says not to test early but if I followed that advice I would not be sitting here in my 2nd trimester!