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Is this something new your cat is doing?
Male or female?
Has there been any changes in the household?
Ok..first, please have your vet examine her to rule out any medical issues (if you haven't already done so) Next, try switching brand of litters. Sometimes its all in the litter. Also, scoop the cat box 2 times per day..some cats get finicky if there is even urine in the box, to where they won't defecate in the box. You can also try getting a larger litter box. You might also try adding a second litter box near where she relieves herself outside of the box. Sometimes that is the best and easiest solution. Please take a look at this very informative website listed below. It will give you some more ideas that might work for your kitty. :) I also want to add to make sure you are cleaning the area she soils with an enzyme based cleaner like Nature's Miracle. This cleaner will not only kill the odor in your nose, but more importantly, it will kill the odor to her nose :)
Hope this helps and hope your baby stops this behavior very soon for you with some of the ideas that are listed :)