how accurate or pregnancy tests (first response)

Are pregnancy tests accurate? 
*Taken correctly
*urinate in a cup and then dip the test in for recomended time
* Use first response and Clear blue digital
*Have been taken them for a few months 
* I take some in the morning some at night 
* Have been taking a test each week, some twice a week
* I started taking them at 4 weeks since I last had sex and have been taking them ever since, I would now be 18 weeks. 
Please give me some answers, I don't want to be pregnant. I have stopped having sex. I know a negative is a negative but I'm worried that the tests might just be wrong because I have weird symptoms. I have gone to the doctor and then said I don't need a blood test. 

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