We adopted 2 cats when they were 10 months old from an owner who loved them very much but couldn't keep them due to health reasons.
The cats were microchipped, vaccinated, had worm tablets and she told us both had been neutered.
At the start of January cats in the neighbourhood started sitting outside our back door and the boy cat kept pouncing on the girl cat, though I never saw them have sex.
Now the female cat is fatter than the start of the month. Is it possible that the neutering didn't work?