Here are excerpts from five websites explaining why it is NOT a good idea to supplement calcium. One of these websites belongs to Merck, another one is where quite a few vets send their clients to read up on different diseases and the other three come up all the time on Google.
Calcium supplementation may be tempting but is not a good idea. As long as the expectant mother is on a quality diet, supplementation is unnecessary. Further, supplementation can suppress her natural calcium releasing hormones so that when she really needs extra calcium during nursing, she will not have the proper hormone balance to get it. This can create a very dangerous situation that could easily be avoided by not supplementing with calcium.
http://www.veterinarypartner.com/Content.plx?P=A&A=1459
Over-supplementation of calcium during pregnancy may increase the risk of eclampsia. There is a complex way the body maintains the proper amount of calcium in the blood. The body is constantly adding calcium to bones and then removing it, as needed. This is regulated by a hormone produced by the parathyroid gland, called parathyroid hormone. If a dog receives increased amounts of calcium during pregnancy, her body's production of parathyroid hormone greatly decreases. When the dog suddenly needs large amounts of calcium for milk production, the system is not ready to start removing it from the bone. This is because it takes some time for the parathyroid gland to start producing the hormone again. Because of the lack in parathyroid hormone, the blood calcium level suddenly drops, and produces the signs of eclampsia.
So, adequate amounts of calcium need to be given during pregnancy, but not enough to slow down the production of parathyroid hormone. This means calcium supplements are generally not recommended. Also, it is important for the calcium and phosphorus in the diet to be at the correct ratio of 1:1 (i.e.; 1 part calcium to 1 part phosphorus). Vitamin D must also be present in adequate amounts.
http://www.peteducation.com/article....articleid=1505
Supplementing calcium and possibly Vitamin D during the pregnancy is likely to lead to problems. This seems to run against logic, since the developing puppies need calcium. What seems to happen, though, is that supplemented calcium is used preferentially by the bitch's body, because it is easily available. So she doesn't maintain normal calcium regulatory mechanisms in top efficiency. Then when it is time to deliver puppies and produce milk, she can't meet the sudden large increase in calcium demand because she has been relying on the supplemented calcium. It is possible to accurately test calcium levels using some laboratory instruments that are reasonable for veterinary practices to purchase, such as the iSTAT machine. If your vet has the capability of measuring calcium it may be possible to determine if calcium deficiency is contributing the problem. Administering calcium without knowledge of the calcium levels is a more dangerous approach to the situation. There is an oral calcium gel (Calsorb Rx) that is supposed to be absorbed pretty quickly and may be less likely to cause problems with excessive calcium (which include stopping the heart).
http://www.vetinfo4dogs.com/dpregnancy.html
The best way to prevent eclampsia is to avoid calcium supplementation during pregnancy and to feed the pregnant bitch a well-balanced, good quality food. Supplementation of the bitch with calcium may be helpful once the puppies are delivered and are beginning to nurse. Supplemental feeding of the puppies may also be beneficial, especially for large litters.
http://www.petplace.com/dogs/eclamps...ogs/page1.aspx
Owners should be warned that this condition is likely to recur with future pregnancies. Steps to consider to prevent puerperal hypocalcemia in the bitch include feeding a high-quality, nutritionally balanced, and appropriate diet during pregnancy and lactation, providing food and water ad lib during lactation, and supplemental feeding of the puppies with milk replacer early in lactation and with solid food after 3-4 wk of age. Oral calcium supplementation during gestation is not indicated and may cause rather than prevent postpartum hypocalcemia. Calcium administration during peak milk production may be helpful in bitches with a history of puerperal hypocalcemia.
http://www.merckvetmanual.com/mvm/in...m/bc/80304.htm