Thank you for reaching out to us.
We are sorry to hear of your nausea and dehydration over the course of the pregnancy.
Based on your description, it is not clear that you would be permitted to fast.
Your first step is to speak with your physician as soon as possible to clarify:

If it is considered safe for you to start the fast
If so, what physical signs would indicate that the fast needs to be broken
If it could suffice in that case for you to eat and drink in shiurim (small amounts), or if you should begin the fast eating and drinking in shiurim.

See here for details about shiurim.
Please get back to us to follow up. For an immediate response, please call our telephone hotline or consult a local halachic authority.
Shana Tova and B’sha’a Tova!