All fmla covers is 12 weeks I believe but if the doctor doesn’t write you a note for the full 12 idk if your employer will accept you taking 12 weeks that’s more of a question for them and their policies.
Legally FMLA is 12 weeks unpaid and employers can not shorten your time or have any repercussions for you when you return to work. Many (not all) companies offer 6-8 weeks of paid leave through short term disability dependent on delivery type.
If you're not going to get paid at all for any time you take off then take the 12 weeks if you want to. I have short term disability insurance and while I'm able to take up to 12, I'll only get paid (at 67%) 6-8 weeks like you said. I think that is what is meant