mom1760, I'm in HR so I'm very familiar with FMLA. If your employer has enough employees so that FMLA applies to them and you've met the requirements (1250 hrs in last 12 months and 12 months of employment), then they do have to allow you to work the reduced schedule/take intermittent FMLA until you've used up your 12 weeks of FMLA, as long as the Dr. provides adequate information. Once you've exhausted the FMLA, though, you will have no protection under this federal law until 12 months has gone by since you first requested the FMLA. Please also look into the Americans with Disabilities Act and you may be able to provide adequate information to show that a reduced work schedule would fall under "Reasonable Accomodation."
Who is saying they will have to replace you...HR or your supervisor? Please talk to HR. Often times, the supervisors don't know the rules and may say something that is not allowed. HR should be able to provide you with more accurate information and, if they are the ones telling you this, walk in there with the laws printed out and tell them that you will report them if they fail to comply.
Crohn's Disease, Acid Reflux/Gastritis, Hashimoto's Thyroiditis, Endometriosis, Arthritis, Depression/Anxiety. Too many meds to list them all. =)