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In reply to the discussion: Hey Cat-Savvy Du'ers -- For the Love of a Cat named Nikita -- a continuation (Part 3) UPDATE [View all]avebury
(11,197 posts)I believe it can be as early as 6 months but some cases earlier.
Most female cats will go into heat by 6 months of age. But certain environmental factors can cause it to happen more quickly. And certain breedslike Siamesemay go into heat as early as 4 months, while for other breeds 10 months old is a more typical age for going into heat.
http://www.catchow.com/Catipedia/Basics/30/When-your-cat-is-in-heat-What-to-expect
I think it is 6 months for a male to reach sexual maturity.
If you can segregate them indoors, you might start with the males first for neutering because that will cut (pardon the pun) the problem off for the females. I would still get the fixed as soon as they are big enough.
How old are the kittens? If they are pretty much weaned and on kitten food I would put Momma at the top of the spay list before she gets pregnant again. A couple of intact male cats started showing interest in the Momma cat that hung around my property. She got spayed (as well as the only baby that stayed around) so they are both safe.