Spaying is technically called an ovariohysterectomy.
Why aren't just ovaries, and not also the uterus, removed in spaying a cat?
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Better to take it all than leave a portion, if you were to only take ovaries you still have the risk of developing a pyometra (where pus fills inside the uterus, causing sepsis and death). If you were to only take the uterus then you would see problems such as them going into heat or obtain various cancers.