I think you have handled it well so far. But if it continues you need to say something. I think I would try something like " Amanda I understand you are trying to work through what happened to you and your baby. It was a tragedy. But you need to stop blaming yourself. And you need to stop trying to scare me. I promise you I am informed of the dangers of various activities and I'm taking active steps to protect my baby. And one of the things I need to do is avoid being needlessly stressed out. From now I'm insisting on if you don't have something positive to say to me about my pregnancy, don't say anything to me at all. Its not fair to me, or to you, to start assigning blame over what happened. I don't think you caused your miscarriage and you need to stop accusing me endangering my baby."
And if she doesn't stop, I think you need to speak to your manager. Amanda is a waitress, not a Dr, and its not ok for her make your workplace a hostile environment, no matter how tragic her situation.
I agree, but I'd go pretty quickly to the "I don't think you caused your miscarriage and you need to stop accusing me of endangering my baby" part.
Also, if she says something in this category again, I would suggest you give her some immediate negative reinforcement. (behavior modification) Do or say something that is less than pleasant--not rude, mind you, but just immediately say, "Excuse me," sort of abruptly and leave her presence. Something along those lines. Or a short, terse, "Nobody thinks you caused your miscarriage, and I'm tired of you suggesting that I could hurt my baby." and then walk away.
Something that will be just unpleasant enough to make her check herself before she opens her mouth.
and I would also suggest that perhaps in general you not be as supportive and understanding. Pull away from her--don't answer her questions at all, or if you do answer them, give a non-answer: "Is that aspirin you're taking?" and you say, "I'm not discussing my pregnancy with you." or "It's not really your business is it?"
Time to make her less comfortable with the casual conversation with you. Because she isn't handling them well, and she needs feedback and tells her so.
But you know what? I really, really like Charlie Braun's incredibly counterintuitive advice. Because counterintuitive is really powerful.
And because all the little "my doctor said" etc. aren't really speaking to the CORE issue. Which is that she's grieving, and your pregnancy is a reminder of her loss, which is why she's grieving like this with you.
But I think I wouldn't say "thank you for looking out for me," but instead just something generally loving: "It's hard, isn't it?"