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Estrus, or heat, is the stage in a female dog's reproductive cycle during which she becomes receptive to mating with males. At this time, estrogen levels first increase and then sharply decrease, and mature eggs are released from the ovaries. read more
How to Tell if Your Dog Is in Heat. Heat, which is also known as the estrus period, is a reproductive stage that unspayed female dogs go through. During this stage her eggs mature, making her fertile and able to produce puppies. read more