When is the latest day in pregnancy which a cat can have kittens terminated?:(

Up until she goes into labor. It won't be nice for the vet and techs if they like animals, essentially the kittens will be under as well and they'll suffocate but won't be conscious they'll be asleep from the drugs like mom, as conscious as kittens normally would be I mean I don't have personal experience with this, but I know that many shelters in the US routinely abort litters no matter how far along they are as an alternative to euthanising the mom as well. What's can gestation? 40 some days? You should call and ask a vet, but probably half way into the pregnancy would be 'humane' because the kittens would not survive being born so young.

Congratulations for asking about such a hard choice, it's not a nice one but sometimes it really is the best one.