I know this is an old post, but since I'm reading it, I'll add my comment.
Yes, I was advised by my vet a couple of years back that Liver is good for a lactating bitch in that it helps with milkproduction.(Beneficial if there is a big litter)
Then I've read somewhere about a study that was done on beagles, feeding liver before welping, vs NO additional liver.
The dam's litter that received the liver was not only stronger, but bigger then the counterparts without the liver added.
I have thus experimented on my last 3 litters, and I can agree. Avarage litter size (per pup that is!) were between 100-200gr more then my previous litters. The puppies were much stronger.
I used to make use of chicken livers, coocked and then chopped fine and added with the normal kebble food. Since I got married and are living on a game farm, I use game liver as that are even more healthy free of any antibiotics etc. (TOTALLY ORGANIC)
The only thing that I cannot figure out, is if it's better to cook it (but kill some of the valuable nutrients) or feeding raw (that is frowned apon). My logic tells me uncooked is better, but the civilised me thinks one should rather cook it.