Since we know calcium is liberated from a mother's bones to build baby's bones and then to make milk it seemed like common sense to have pregnant and nursing women supplement with large doses of calcium. The problem is, after decades of that advice we're learning that women who supplement actually liberate more calcium from their bones and that after weaning their baby women naturally put down extra calcium back into their bones leaving them stronger if they are taking in normal (dietary) amounts of calcium. That's amazing! Your body uses your own calcium to grow your baby, then after about 6 months of nursing begins laying down stronger bones in preparation for your next pregnancy.
A large Harvard study of nurses backed up these findings, showing that those who took a calcium supplement broke bones more often than those who didn't. Almost as if your body knows what it's doing and adding mega amounts of one mineral upsets the delicate balance your body is constantly working to achieve! This is a case of more is not better. Too much is harmful. It is important to remember that it is not only calcium that our bones are made of, even though it's the one we hear about most often. Protein, magnesium, sodium, and phosphates are equally important in bone composition.
Getting your daily dose of around 1200 mg calcium shouldn't be difficult if you're eating a healthy, balanced diet like you should in pregnancy. Calcium is in many foods, from dairy to many vegetables. It's also added to many foods, like orange juice. In fact, dark leafy greens, sardines, and nuts have good amounts of calcium. Another thing to know is that your body seems to only absorb 500mg of calcium at a time, so it's best not to take all your calcium in one supplement.
Unless you have a problem absorbing calcium, you're not eating a healthy diet (in which case you'll be deficient of many nutrients), or some other reason your doctor thinks you need to supplement, the evidence is pretty clear that women, even pregnant and lactating women, don't need extra calcium supplements.