FMLA extension

Ive used up 8 weeks & wanted to use the remaining 4, I thought I could call HR & just get the extension but I was informed my doctor would have to write a note stating why I need to take off the other 4 weeks. My reason is I won’t have childcare around the time I’m scheduled to go back so I need the additional month to work with her fathers schedule. My problem is I don’t think a doctor will sign off for that reason. What are some reasons a doctor would extend FMLA leave