FMLA and Maternity leave

So I work for a non-profit that is subject to FMLA but doesnt offer any paid maternity leave. My HR rep says I have to come back to work after my doctor clears me to work at 6 weeks postpartum (10 for c-section) but my husband argues that FMLA states I can choose to stay out for the whole 12 weeks. My HR says that's not the case and my coworker who has had a child during her time working there said she had to come back after 6 weeks. Even my OB said that yes, that is up to the company whether I get more than 6 weeks. Does anyone else think that's not what the FMLA lae says? I know it says "up to 12 weeks" but who decides how long?