Just as parent's hours sometimes change at work with promotions, changing jobs, etc. providers sometimes need to make changes for their family needs too so it works both ways. When either party in the contract needs to make a change they need to give at least the minimum notice as specified and it then makes the contract void. Meaning that if the new hours don't work then the parent will need to move to a caregiver with more suitable hours. A contract is there for protection but is subject to change as circumstances change. Fees can be changed as food/hydro prices go up, hours can be changed as provider's children age and begin activities. Sometimes even the days of childcare change as a parent decides to go part time over full time.
There is no such thing as standard hours for home care and you will find several caregivers all on the same street with different hours. Care ending at 5:00 is the most common in most areas. When you have all of your other families picking up earlier and staying open for just one it doesn't make sense if it means you have to jeopardize your own family's activities and that is why the changes were made.
As far as the pregnancy goes I think the provider just assumed you would either keep the child home or have the ability to work within the shorter hours come Sept - if you are home on mat leave why are you not able to go pick up your older child by her new closing time and then negotiate with your work when you are ready to go back to find a job with hours that work for daycare. It doesn't always have to be the provider that adapts.