The MG Manual address pregnancy issues:
Myasthenia may improve, worsen, or remain unchanged during pregnancy and it is not uncommon for the first symptoms of MG to begin during pregnancy or postpartum.
Myasthenia Gravis: A Manual for the Health Care Provider (PDF) - page 25
The MG Manual is free for download. It is a PDF document and provides many answers about MG.
If you search the MG Forum for "pregnancy" and "pregnant" you will read posts on the topic.
-Mark-