I like Danfresnourban's answer: write a letter and ask if you can get a discount off the tests explaining why you think this should be the case (but ultimately just pay the amount they decide you should owe). NOTE: You need to make folllow-up phone calls too to lobby for this, don't just rely on 1 letter. Talk with whoever does the billing, as well as the physician if necessary. While the process is being decided, ask the billing office for a (free) extension of the due date on your account, or at very least request they enter a note on your account explaining that some of the charges are currently under review.
However, in my view, a wider issue is at large here that Ella also raised: if you don't have confidence in this doctor, you might want to consider switching to a different doctor, and fairly soon... in the US, a lot of doctors prefer their new obstetric patients to be less than 19 weeks along (although in some cases it may still be possible to switch later). I actually switched to a different practice group about 18 weeks along during my 1st pregnancy because I didn't have a lot of confidence in my 1st doctor after they missed timeframes for prenatal tests and had problems scheduling my prenatal appointments... ugh. Good luck with everything! Feel free to ask if you have more questions about this!