I just had my 7 month old male cat neutered 4 days ago but now his sister has gone in heat and he is chasing and mounting her.
Since he is only newly fixed can he still get her pregnant?
Should I lock them away from each other?
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He can impregnate her for a short period after neutering.
Separate them from each other at this early stage.
When a cat is neutered the testicles are removed, making the cat unable to create sperm. The residual sperm can still impregnate a female cat. A search shows different time frames for which this residual sperm can still be viable and impregnate a cat. The ASPCA suggests 30 days, Catster suggests a few days and another source suggested 6 weeks. If you ask your vet, they should give you direction about this.
As for the behaviours, it takes some time for these behaviours to settle down, but in such a young cat, it's likely they will.
... Male cats must be separated from any females on heat for one week after the operation as there may still be viable sperm in their lower passages that cannot be removed by surgery. Only after this week has passed will male cats not be able to sire any kittens.
Keep Your Pet Away from Other Animals.
Keep neutered males away from unspayed females. Neutered males can get an unspayed female pregnant for up to 30 days after spay/neuter surgery. Keep spayed females away from unneutered males who may wish to mount them. Animals returning from the Mobile Spay/Neuter Clinic may also smell different to other animals in the household. This can cause the animals to fight, so be prepared to keep your pets in separate areas for a few days following surgery
... After the testicles are removed, it takes several days for all of the residual sperm to clear out of the pipes. During this time, a freshly-neutered male cat can still impregnate females.
It can take more than a week for the male’s testosterone levels to drop. During this time he will remain at increased risk of fighting, spraying, and engaging in other undesirable (from our point of view) behaviors.