
I have always gradually increased to 1/2 as much again as maintenance from 6th week on. If by 8 weeks they look like it's going to be a large litter (6+) and they seem hungry I might increase to double by whelping.
After whelping I feed ad lib and find they are on about 4 - 5 times their normal diet until pups are abut 5 weeks, then I decrease the food a little so by 8 weeks they are on just double rations.
I find with my coated breed they need to stay on double rations or sometimes 1 1/2 until their coats come back as otherwise they loose condition during the post partum moult.
The earliest I have had them showable is 4 1/2 months after the litter, but generally not until around 6 months after they have their next season.
So in my breed you loose a lot of time out of the ring with a litter.
My girl who had pups in September has been out of the ring since July and her first who will be the end of April. she has just come in season a few days ago.