Lol... The smell of cat urine or feces will not cause miscarriage or birth defects. The worry is toxoplasmosis which is a micro organism which can be contained in some cats with no signs. I have 6 cats and am a veterinary technician. Have someone else clean the box or wear gloves. You will be fine.
You can have cats you just can't clean the cat box.
The smell of ammonia in anything can cause birth defects and in some cases miscarriage.
And there is some sort of bad stuff in the poop too not just the urine. Just have your s.o.clean it and often
It can be dangerous to the baby because of the ammonia. As long as it's litter box is cleaned on a regular basis you will be fine, and maybe have your S.O. take over the litter box cleaning while you are pregnant.