M.K.
What Courtney (and some others) said in the very first post is true. No company in the US is REQUIRED to offer employees maternity leave at all. The custom of "6 weeks for a regular birth and 8 for a c-section" is a common benefit that many companies (especially publicly-traded Fortune 500 companies) tend to offer.
The only Federally protected benefit you may have, depending upon the size of the company you are with and other factors, is the Family and Medical Leave Act (FMLA). If your company must abide by FMLA, you are entitled to 12 weeks, unpaid leave. They do NOT have to give you back your current position -- they can put you in a "comparable" at the same pay rate.
If you have an anonymous HR line, call and ask about your company's policies on maternity leave. Be aware that HR function is to support and protect upper management...so if you are not having an anonymous or protected conversation, you can never be certain of what will be reported back to your Manager.
Most likely, you'll probably need to work with your Manager to put together a leave plan that includes (1) any maternity leave they offer, plus (2) use of any paid vacation or time off you have accrued, and then (3) FMLA time. Unfortunately, you cannot control her reaction and attitude about losing an employee for an extended period of time. I hope she is not mean to you...but please document ALL conversations with her immediately after having them, to keep on file. Maintain your professionalism and your positive attitude, no matter how she treats you. If you feel mistreated or discrimination, you will need this documentation. Personally, I would follow-up every single meeting about the topic of your pregnancy with an email outlining the instructions or feedback she has given you.
Finally...CONGRATULATIONS ON YOUR WONDERFUL NEWS!!!!! May you be blessed with a healthy, easy pregnancy!!