Before deciding to resign, I would strongly recommend that you consult with an plaintiff's employment attorney licensed to practice in Washington, DC. You may have claims that you could pursue with the D.C. Office of Human Rights, the U.S. Equal Employment Opportunity Commission, and/or the U.S. Department of Labor. You can find such a lawyer through mwela.org's find-a-lawyer search function, or you can contact my firm for a consultation.
You might have 2 causes of action based on the facts that you have described. 1) under Title VII and the Pregnancy Discrimination Act - this for retaliating against you by giving you a poor performance review after the birth of your child and 2) again for retaliation as under the FMLA.
I would not recommend quitting, as the case for "constructive discharge" is relatively tough to prove, and this may jeopardize your case, in the event that you decide to file a Title VII claim against your employer. I would rather the employer fire you, and then proceed from there. Also with the latter you will be able to obtain unemployment insurance, but not when you quit.
Finally note that you have 300 days from the date of discriminatory and retaliatory activities to file a Title VII claim. The information can be found here: http://www.eeoc.gov/employees/charge.cfm
You can contact my office if you need help