Low Birthrate Kitten-prognosis?

My cat gave birth to an underweight kitten a week ago (just over 2 oz). I knew the chances of the kitten surviving were slim, but since it seemed to have no problem nursing and the mother didn't reject it I decided to let it stay with it's mother and two normal weight siblings. 6 days have passed and the kitten is still alive and nursing and has even gained weight (although it is still about 1/2 the size of its siblings). Are we out of the woods? I didn't go to a vet because the kitten has been behaving and eating fine and the only obvious issue is the low birth-rate.

(ps-I am caring for the mother and kitten as a foster because the mother was in a shelter and already pregnant. I do not breed and will be having this mother spayed as soon as the kittens can be left alone for a few hours. Right now she is being kept away from intact males.)

I can just tell you your cat will most likely be fine. It will always be smaller than the other kittens, but he'll live.

P.S. HAPPY EASTER!

You're not quite out of the woods, but your tiny kitten sounds like it's doing just fine: that it's nursing, putting on weight, and the mother is caring for it are all great signs.

Still, I would keep watching tiny kitten at this point, ensuring the queen doesn't reject it, and that it keeps putting on weight. Tiny kitten should have roughly doubled it's birth weight after 2 weeks (gaining 7-10 grams per day, average: some days maybe less, some more.)

If tiny kitten
- starts vomiting or diarrhea
- starts crying constantly
- starts sleeping separately from the queen and siblings
- loses or stops gaining weight
contact your vet RIGHT away.

After 2 weeks of gaining weight, tiny kitten's prognosis will be good! My cat (now 8 years old) was very much the runt of his litter: we didn't think he'd survive when he was born, but now he's heavier and smarter than his big brother!

Watch the baby very carefully, and make sure he's gaining weight. Are you weighing the kitten daily? A food scale is great for this, as it's designed to weigh in ounces. As long as he's growing and nursing, and not experiencing very loose stools, he will probably be fine. Usually if the mom doesn't reject the kitten, she doesn't "sense" anything wrong with it. I've had a couple runt kittens (same circumstances as you have now - I do NOT breed) and many of them catch up to their bigger siblings eventually.

Cats and dogs usually know best if a baby is defective and not going to survive. They usually will separate it from the rest and just let it die. Since the mother is still nursing the kitten, then it's probably going to be OK.