Pregnancy - Calcium

Hi,

I have a female JRT who is five weeks pregnant right now and this is my third litter of puppies. I have previously included a small amount of powdered egg shells (about 1/4 tsp) in my girls diet to add some calcium to ward against eclampsia, she'd also get it while nursing. Also, another breeder recommended using half a calcium pill. But, I'm in the process of reading "Dog's: The Ultimate Care Guide" and it mentions to be very careful with suppliments, like calcium, and that they could cause deformities. It gave me a scare so I was wondering if you have any advice/experience. My pregnant girl is 5 years old, this is her first litter and she is now on Purina Growth plus wet dog food.
Thanks in Advance!!