First stage can last from 12 to 24 hours. http://www.quality-cat-care.com/cat-labor.html
If it goes longer than 24 hours, or if it goes into second stage and then returns to first, call the vet. Otherwise I'd make sure she has access to water, because of all the panting, let her try to mother her grandkittens (as long as her daughter and the kittens aren't complaining), and be available to her if she needs you. She's obviously fully into mothering mode; I'm sure she'll be a very good mother. Also, it's obvious that she's done this before or she wouldn't have grandkittens, so she really should be okay. She knows what she's doing. I suspect that you're more anxious about it than she is.
It could be Braxton Hicks contractions, though I'm inclined to think this is the real thing. See http://www.thecatsite.com/t/306346/can-cats-have-braxton-hicks-contractions
Since part of first stage is simply getting the litter into proper position for birth, and she obviously has either a very large litter or a litter of very large kittens, that may be part of what's slowing things down. It was much faster with Sweet Thing, but she only had four to deal with. Also, I wonder now whether she didn't go through first stage with me totally oblivious.
Sleep well.
Margret
If it goes longer than 24 hours, or if it goes into second stage and then returns to first, call the vet. Otherwise I'd make sure she has access to water, because of all the panting, let her try to mother her grandkittens (as long as her daughter and the kittens aren't complaining), and be available to her if she needs you. She's obviously fully into mothering mode; I'm sure she'll be a very good mother. Also, it's obvious that she's done this before or she wouldn't have grandkittens, so she really should be okay. She knows what she's doing. I suspect that you're more anxious about it than she is.
It could be Braxton Hicks contractions, though I'm inclined to think this is the real thing. See http://www.thecatsite.com/t/306346/can-cats-have-braxton-hicks-contractions
Since part of first stage is simply getting the litter into proper position for birth, and she obviously has either a very large litter or a litter of very large kittens, that may be part of what's slowing things down. It was much faster with Sweet Thing, but she only had four to deal with. Also, I wonder now whether she didn't go through first stage with me totally oblivious.
Sleep well.
Margret
