1. as two things are simultaneous: pregnancy
+ HCG injection
, thus it will pose some difficulty in determining levels of HCG i.e. whether due to pregnancy or injection!
2. time gap of 12 days is sufficient for clearance and metabolism
of injected HCG, thus do keep this thing in mind.
3. in early stages of pregnancy, HCG levels doubles every 48-72 hours, if HCG levels are rising slowly or dropping , often an indicator of miscarriage