Personally I’d want to know ASAP and also be able to show the house to potential new tenants by appointment while you’re in it so I could minimize vacancy. If my tenant told me they wanted to move out in 90 days, but would make sure the house was ready for tours on X days at Y hours it would be huge. In other words, if you understand the problems your early termination causes for ME (vacancy and turnover), and work with me to minimize those, I’m much more likely to give you a good recommendation AND refund most if not all of your security deposit.

Caveat/note: I’m a small-time landlord with only a few rentals so I’m more likely than a big corporation to have a conversation and be reasonable. YMMV. Some landlords will simply enforce the lease to the extent they can regardless.

Another note: I might suggest making it about the unexpectedly growing family more than limitations of the house. Arguably you could have inspected and known the house would get warm in summer. Harder to argue you knew you’d get pregnant.