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Hello my name is***** and I am a veterinarian with over 9 years of experience. I'm hoping to help answer your questions regarding your pet today. Since I can't perform a physical exam on your pets, I may need to ask several follow up questions if you don't mind. Are you planning to use your females for breeding? How many heat cycles have they had already? Was there last heat cycle obvious with enlargement of the vulva and a fair amount of bleeding?
Ok thanks for the additional information any problems with any of the litter? How big were the litter sizes?
Ok great there are a couple of things you can rule out to help determine the cause for the delay in heat cycle. One is if there is an hormonal issue. Sometimes dogs can have a problem with a thyroid insufficiency that can cause a delay. A silent heat cycle is also a possibility as well. Blood tests can easily rule out a thyroid issue. The silent heat may be a little more challenging because it would require measuring progesterone levels and if they are high then that would confirm a silent heat if they are not showing any outward signs. But if normal it just indicates that they are not in heat.This can be done in conjunction with a smear of the vaginal cells which can determine what part of the heat cycle they are in
The other thing to rule out is if it is an anatomical issue especially with the ovaries. Sometimes cysts on the ovaries can cause delay. An ultrasound can help rule that out
If those are determined to be normal then you would want to rule out a Brucella infection which specific tests are available for
I thought your question was what you can do to determine why your dogs are having a delay in a heat cycle. My suggestions were ways to determine a potential cause or causes that may or may not have to be addressed. The only other option would be to wait and watch. Also you if the females are siblings you may not have to test all of them for these issues. Sorry you were not satisfied with my answer I will OPT out so another expert can help you