I wish you luck with your doctor.
If it were ME, I'd be pretty clear on how to present your situation before going in... unless you have immense trust and respect for your doc, or already feel they will be supportive, this could turn out to be quite the uncomfortable situation.
I don't know where you live (if you're in a med-pot state/country, etc.) or anything like that, to know what the general climate surrounding pot-use is, let alone for expectant women with medical concerns.
I will say that I had quite an advantage for the following reasons:
1. I live in a med-pot state with generally accepting attitudes surrounding cannabis use and relatively relaxed cannabis laws.
2. I have a well-established history of pre-existing medical problems, as well as a history of having my medical marijuana recommendation
3. I am using a Licensed Naturopath as my primary care physician as allowed in my home state
4. My ND is a cannabis advocate, she is also a professional researcher by trade and is well-versed in the evidence (or lack thereof) of fetal harm
5. MY ND was a colleague of mine; we worked together thus I already knew (roughly) what her position would be. Also, since we worked together, she knows ME and was well-acquainted with my level of responsibility, maturity, etc. I guess what I'm saying here is that she already knew I was a med user and that I am a high functioning responsible user
The reason I lay all this out here is because I've read stories of women revealing their use to their doctor, only to be made to feel like "druggies" harming their unborn babies! Depending on a LOT of factors, your conversation could go quite differently than mine and you should be prepared for this possibility.
The LAST thing I'd want to see would be anyone taking my experience as "typical" and end up with a CPS case!
I do believe women should be able to be open and honest about their cannabis use within their patient-physician privilege (there is no shame in medicating with less harmful substances to treat valid conditions), but I just fear this isn't always the case.
Educating yourself in advance with whatever research you can find is surely a step in the right direction.
Again... BEST OF LUCK and CONGRATULATIONS