I have just bought an eight week old rottweiler puppy, and would like to know roughly how much she should weigh as she seems quite small and underweight, please can you help me?
She was from a litter of 9 pups. could this be the reason she is so tiny ?
Even if they're the same breed, puppies can vary a lot in size, different bloodlines/parentage produce different pups. It's also quite possible for puppies within the same litter to vary considerably.
There is usually a 'big' puppy, a 'small' one (sometimes called the runt) and then the rest fall somewhere inbetween. This variation in size can be due to physical differences, such as bone size, or simply due to nutritional inequalities while the puppies were in their momma's womb.
The 'runt' (unless it has inherent health problems) doesn't necessarily end up as the smallest of the litter once the pups have grown up. Likewise, the largest at birth may simply be the one that got the 'lions share' of in-utero nutrition, and may not be bigger than his/her siblings when adult.
Many Rottweilers with German/European dogs present in the first or second generation of their pedigree are bigger boned and heavier than those with purely American bloodlines. However, different bloodlines (even among German dogs) can produce dogs of different heights, weights and body types.
My dogs are of German/Yugoslavian lines, weigh around 120lbs as adults and have big bones and solid structure. An 8 week old puppy from my dogs usually weighs around 12lbs. This is considered a large pup, and it's important that the weight be attributed to bone size not fat!
An 'average' weight puppy produced by parents who are lighter boned or smaller than my dogs, may weigh around 7 - 8lbs at 8 weeks old. This is simply due to the size of parents and ancestry, it doesn't necessarily mean the puppy is inferior in any way, just smaller.
If your puppy seems happy, active, eats and drinks normally, has bright eyes and a shiny coat, chances are she's just fine, even if she seems small. However, if she seems 'bony' (ie you can see her ribs and she's not got that layer of 'puppy fat'), seems tired, or has any diarrhea, vomiting or other signs of illness, I would recommend having your vet take a look at her.
Rottweiler puppies of 8 weeks old should look 'cobby', ie kind of square. They shouldn't look lean, but neither should they be overly fat. Your pup was from a reasonably large litter, and it could be that she was the smallest, or maybe the least forceful when nursing. Either way, as long as she's healthy you've no reason to worry.
Do make sure that you feed her a high quality puppy food, specifically designed for large breed puppies, and get all her vaccinations and deworming done on schedule. Rottweilers are especially vulnerable to the dangerous Parvo Virus and she will need to be protected from this asap.
Hope this puts your mind at rest. Best of luck with your puppy.