I dont have paid medical coverage for maternity leave at my job. My employer threatened my job today, she stated she is hiring a full time employee since I cannot tell her 100% if I will be returning or not. My boss stated by law she only needed to hold my position for 6 weeks. Is this correct?

We need a little more information before we can say if your boss is correct. Where do you work (what state), how many employees does your company have, and how long have you been working there? Also, does your company have an employee manual, and does it have policies about taking leave for medical reasons?

depends on employer size, your status, length of employment, etc...

fyi on pregnancy discrimination http://www.eeoc.gov/facts/fs-preg.html