Why not take the lower job, and look for a new job. If you resign, you will miss out on Unemployment.
I represent Employers, but I can recommend Worker Attorneys in So Cal if you ask.
The red flag in your question is that it is unlawful to discriminate against an employee because of her pregnancy or pregnancy related leave. FMLA protects employee so that they must be returned to their job or an equivalent position UNLESS they would have been laid off, etc. regardless of the FMLA leave. Call an employment law attorney to discuss. Many offer a free initial phone consultation.
This is a very personal choice and only you can make it, but you should consider medical benefits, whether you will be able to collect unemployment based on the elimination of your position, whether there is a benefit to you & your future employment to stay, if there is a chance you can move back up, is it a company with a good reputation, do you like your job, company, & colleagues? On a sheet, draw a line down the middle and list pros & cons of leaving & pros/cons of staying. When you have your chit chat with Boss & HR, ask how an unemployment application will be handled if you choose to leave. A big consideration with a new baby would be medical coverage if you are not otherwise covered, COBRA can bankrupt you. As for whether you were discriminated against based on your pregnancy, ask yourself whether the position would have been eliminated if you had not been on Maternity leave. If it would have been regardless, it would be tough to prove discrimination. Good luck!
Lesly J. Adams has been licensed to practice law in California since 2010. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, consult with your own attorney.