FMLA-maternity leave

I spoke to my HR department and I'm wondering if this is normal. My employer will pay my full salary for 6 weeks after birth (8weeks with a c section). She said I am allowed a max of 12 weeks off under FMLA but I must use my vacation after the first 6/8weeks before I can take time off unpaid. Is that normal? I'd rather take some time off unpaid and save my vacation in case baby is sick or I need off.