Drug test threats

So at my first appointment with my obgyn I was drug tested, but they didn’t say anything to me about the fact they were going to do it. I am sure I signed something in the stack of paperwork that said they could, and I have never touched drugs so I wasn’t bothered till I got the bill from the lab.... my insurance doesn’t cover drug tests for any reason and I was charged over $500 for this. When I asked my office about it she said that they were going to do another one at 35 weeks and one more at delivery. I told them I wanted to sign something declining these tests and she said that would be considered suspicious behavior and they or the hospital would end up calling DHS to start an investigation into me. I passed the first drug test and work with children where I can be randomly drug tested at any time so I have no issues with the test, but we are not in a financial position to shell out $1500 in unnecessary testing. Should I call their bluff and refuse? If DHS gets involved will they pay for any drug test they want me to take? Also is this normal for all OBGYN offices in the Knoxville, Tennessee or just mine? Is 21 almost 22 weeks too late to switch Doctors? I have only met the actual doctor once and it has been the PA ever since so I’m having a lot of trust issues at the moment with these people, but I really want to deliver at this hospital. I really just feel like I am being bullied and I don’t like it 🙁.