Question: 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?

Answer: 
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.

[From Greencross Vets](https://www.greencrossvets.com.au/aftercare/dog-cat-desexing-aftercare/):

> 
> ... 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.
> 
> 
> 

[From the ASPCA](https://www.aspca.org/sites/default/files/upload/images/caring-for-your-cat-or-dog-after-surgery-1.pdf):

> 
> **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
> 
> 
> 

From [Catster](https://www.catster.com/lifestyle/can-freshly-neutered-males-still-impregnate-females-and-why-do-young-cats-suddenly-perish):

> 
> ... 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.
> 
> 
>
Yes, a [neutered male cat can still get a female pregnant](https://learnaboutcat.com/can-a-neutered-male-cat-still-get-a-female-pregnant/). Your male cat's genital tract will still contain testosterone and sperm for **at least six weeks after neutering**. Therefore, he is still capable of impregnating female cats. If your male cat has recently been neutered, **you should keep it away from your female cat for almost 30 days.**

After being neutered, neutered cats will no longer produce sperm, so they won't engage in sexual activities. He also loses interest in mating because his testosterone levels are significantly diminished.