I have given this a lot of thought, don't worry.
Including subtracting the time off for our cruise, I will have about 7 weeks of vacation time saved up by October. I plan on needing 3-5 weeks of that vacation time for my maternity leave. The amount I will need in vacation time will depend on if I have a natural birth (6 weeks medical leave) or c-section (8 weeks medical leave). I would prefer natural, but we will see how things go. The medical leave qualifies to be paid from my extended medical leave balance at 100% salary. I am not sure if I accrue vacation time while on medical leave. This is still something I need to check on.
There will be also be several holidays during my maternity leave which will allow me to use less vacation time. I get Thanksgiving and Black Friday as paid leave (though I may still be on medical leave at this time, so I am not sure if I get the vacations or not). I also get Christmas, the days between Christmas and New Years and New Years day as paid leave. This saves me about 1-1.5 weeks in vacation time during that period.
Also, my normal sick leave resets in October, so I will have that throughout the year if I need to care for the baby when ill. I get 14 days of sick leave each year. My husband also gets the same sick leave too, so if necessary, he could use his. And finally, I accrue vacation at a rate of 2 days per month even while taking vacation time, so I should earn 3-4 more days of vacation during my leave and even more if I still accrue vacation while on extended sick leave.
So that would mean that I should have another 3 weeks or more of vacation time even after my maternity leave is over. And I may consider discussing with my boss the idea of starting back as part time over that first month of work, maybe 2 days, then 3 days, then 4 days and finally 5 days, just to help me ease back.