Ladydogs can come into heat any time from 6 months onwards.
You will have to be careful with your male dog even though he has been castrated, as he may still try to mate your b itch, and given the size difference, he could hurt her quite badly by treading on her etc. He is unliukely to become aggressive but he might well become frustrated and howl, as well as trying to get at your b itch if he is shut away from her (which you will have to do).
You have plenty of time to plan for this, so well done for thinking about it now If your home isn't big enough to keep them separate, you might like to find somewhere you can send your male dog for the 3 weeks of the heat, and get him used to the new place before he has to be away for the whole 3 weeks.
chihuahua
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Re: chihuahua
A dog is never bad or naughty - it is simply being a dog
SET YOURSELF UP FOR SUCCESS
SET YOURSELF UP FOR SUCCESS
Re: chihuahua
Nettle has also written a brilliant post here about what to expect when your girl comes into heat. You can read up about it all here: viewtopic.php?f=6&t=10933