It will be nine weeks from the first day that you had your last period. If it's been longer since your last period, you are further along in the pregnancy. If your last period was sooner, then you are less further along.
If you don't know when your last period was, then it's best to be honest with the midwives (not guess), and they can refer you to have an early scan, which will be much more accurate in working out where you are in the pregnancy.
9 weeks pregnant does not mean "you have been pregnant for nine weeks".
So assuming you had totally average cycles, this would mean that you got pregnant roughly seven weeks ago. But you don't get a positive test the moment that you get pregnant - it can take up to two weeks or more for your hormones to get high enough for a test to detect it.
So say your hormones took two weeks to get high enough, that would mean that you would start testing positive roughly five weeks ago.
But all of this is assuming you had very regular standard cycles.