Your boss doesn't have it technicially correct. When you go on maternity leave following the birth of your baby, that's like an extended sick leave, as opposed to "quiting". This allows you to take either 6 weeks for a vaginal birth or 8 weeks for a C-section. That is why you apply (and are automatically granted) disability from the state, which amounts to a percentage of your salary (which is calculated from the last two quarters that your work and I think is something like 55%). If you have extended sick leave benefits through your work, you can take that time in order to continue getting paid during this time. This is when you are also receiving disability benefits through the state.
Following the completion of your maternity/sick leave (which is either 6 or 8 weeks), you are also eligible for FMLA which is the 12 weeks that your employer is referring to, which is federal time. The two leaves are separate. The state also provides for six weeks of benefits called bonding leave, which is an additional 6 weeks of paid benefits during your FMLA leave. Bonding leave is a California state benefit that your boss may not be aware of. Call the state disability office for more info-the other posts also have the website. With the two leaves combined, you should be eligible for up to 18-20 weeks of leave depending on your type of birth.
I was able to take a month off prior to my due date (unpaid sick leave which I used my extended sick leave benefit to cover), and took 3-1/2 months off following the birth of my daughter during which I used up the remaining sick leave and my saved up vacation time. I couldn't use vacation time to extend my leave beyond the 12 weeks of FMLA but I could use it to get paid while on leave. Hope that helps. I happened to be working for a big corporate place where the whole thing was automatic. But the bonding leave benefit from the state is relatively new. Take care and good luck with the coming adventure!