NO MORE KITTEN SEASON.ORG
Humans Have
Birth Control
Why Don't Cats?
We are mammals and cats are mammals and squirrels are mammals and hippos are mammals. We all have similar reproductive parts and similar hormones that control the stages of reproduction. It shouldn't be too hard to figure out a low-dose medication to keep cats from having accidental babies, in the same way that there is a very low-dose medication to keep humans from having accidental babies.
It's All About the Eggs
The easy way to prevent cat pregnancy would be to keep the female's eggs from
Maturing or
Releasing or
Fertilizing or
Implanting.
Any one of these would be enough to prevent the next litter of accidental, homeless kittens, before they start. All it takes is a very low dose of medicine that nudges the egg system a little off course.
A mini-dose would not affect any pregnancy already implanted in the uterus. Any kittens started before the contraceptive treatment would continue on to be born.