The cats are sensitive to change, and some become very protective of their expecting female owners, while others become anxious and aggressive.
The fear of change is one of the reasons why cats are aggressive toward pregnant women. They are creatures of consistency and hate change. They can sense that something has changed and that their attention might be diminished while their human is pregnant.
If he doesn't receive the personal attention he's accustomed to receiving, he may shake things up - possibly literally. He might feel neglected, and he might feel angry. After all, he has been your faithful companion for so long, and now he feels snubbed
A cat's territorial aggression is usually directed toward another cat but can also be directed toward people. Any changes in their world could cause negative behavior. They may start to think that their role within the home is beginning to dwindle or that they are not getting enough attention.
Feeling like your beloved companion suddenly hates you during pregnancy isn't comforting to someone preparing to care for another tiny baby. You should see some improvement as soon as they are able to adjust to these new changes if you give them the time and space they need.