The feral I'm adopting won't use the litter box?

She is 1 year and 2 months old. First I saw her with her mother and sisters/brother when she was 1 and a half month old, and since then I've been feeding her, in april however I noticed she was pregnant and in first week of may she gave birth, inside our house. Since then we've locked her inside and she only goes out to do her business, because she wont use the litter box! She used it three times, that's it.  Our vet did not agree to get her spayed as long as her breasts are full of milk so we could have an issue here soon. She knows where the litter box is, she has used it three times but she is no longer using it. I have cleaned it after every use, scooped off the wet litter and added fresh one.

The litter box is located near our toilet seat, because she had peed there before we got the box so we thought it was a better spot. Could that be why she is not using it? She used it three times in exactly that spot. She simply will not use the bathroom if we do not open the door. She will come in the room, meow, then ask us to open the door. I once didnt for 7 hours hoping she would use the litter box, she didn't. She got stressed cause she was obviously holding it. For 7 whole hours just not to use the box!!! Thankfully she didn't eliminate elsewhere. Good kitty

Any idea what I can do to make it more attractive to her?

Update: Forgot to mention. She only peed twice in the bathroom next to the toilet then we put the box there, she used it 3 more times in the first 4 days after birth since she didn't want to move away of the kittens. Since she got stronger, she goes out all the time.

Have you tried Dr. Elsey's Cat Attract litter?  It works really well to attract cats to the litter box. 

Also you might want to move the litter box to a different location or try having more than one for awhile.  She may not like the location.  I would leave the one in the bathroom and place another one in a private location she can easily access.  Definitely try the Litter Attract.  I have used it on all my ferals and they catch on very quickly. 

Also be sure the litter box is not covered.  She may feel trapped inside if it is.

As soon as she is done nursing, you will want to see about spaying her.

Omg she did it! She asked to go out, it was raining so she stayed inside even though we opened the door. Then I heard the litter moving. And she did it! She used it. I don't remember the last time I was this happy lol, it may sound weird but I am almost celebrating. This is the first step to her becoming a good home kitty, I hope

The Catsage said:

 
Omg she did it! She asked to go out, it was raining so she stayed inside even though we opened the door. Then I heard the litter moving. And she did it! She used it. I don't remember the last time I was this happy lol, it may sound weird but I am almost celebrating. This is the first step to her becoming a good home kitty, I hope

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      You're in good company here.  I think everyone will join in your celebration.