i suggest u rush her ASAP to a good vet pregnancy in dogs cannot cause seizure , yes! a depletion of vital nutrients in pregnancy may be a reason , there may be various causes of fits ,but only a vet can make the best diagnose , and yes a seizure is dangerous during pregnancy . During a seizure all the muscles contract, the uterus is a muscle and a seizure would definitely put the pups at risk by contracting and squeezing the pups either killing them or putting them in dire stress. It would also restrict the blood flow bringing oxygen to the pups. take care of her and rush her to the vet , good luck !