Hello Ursula and thanks for bringing your valued question to the Disney Parks Moms Panel.
I absolutely love that you're already planning a magical holiday next year to Walt Disney World Resort for your beloved, but as yet unborn little Mouseketeer, it's just too adorable!
I'm happy to share that in my own extended families recent experience it is indeed possible to add an infant to an already confirmed Disney holiday booking.
When little one arrives I recommend speaking to knowledgeable Cast Member who will be able to assist further by dialling the telephone number for
Disney Resort Hotel Reservations contained in this
Walt Disney World Resort Phone Directory; be sure to have close to hand all the relevant Disney confirmation numbers and so on.
For your added information I also thought you may find these
Disney Resort Hotel Reservations - Frequently Asked Questions to be an extra useful planning resource.
If I can help sprinkle any extra pixie dust onto your jolly holiday plans to visit Walt Disney World Resort in 2018 please do pop back to ask me another question.
Have a great Disney day.
~ Jane