During pregnancy your body changes in many ways to supply your baby with calcium before and after birth. Your intestines absorb more of the calcium from your food, your kidney excrete less calcium in your urine, and calcium is released from your bones.
It's this last effect-calcium being released from your bones-that caused many women and their care providers much worry. It seemed alarming that a pregnant and/or lactating woman was losing her bones' calcium into her blood and, ultimately, her baby. Wouldn't this cause her bones to be brittle and even give her osteoporosis as she got older? Once again, careful study of the facts has restored our faith in the normality of pregnancy and lactation. Studies show that a woman's bones are replenished with their lost calcium within about 6 months of weaning no matter how much calcium was lost. Not only was the calcium replaced, but the amazing body laid down extra calcium. That's right! Women who had more babies and those who breastfed had denser bones! Furthermore, there is probably a buildup of calcium in mother's bones during the first trimester, ensuring a supply before the fetus gets very big.
Of course, this replenishment, early buildup, and conservation of calcium all depends on adequate dietary intake of calcium. But how much is enough? The recommendation right now is 1 g/day. That's only 1 cup of plain yogurt, 1 ounce of mozzarella, and 1/2 cup of ricotta a day. We can all do that, right? There's a complete list here of the calcium in various foods , because while dairy is a good source of calcium, it's far from the only one. Remember, calcium from food is absorbed better than that in supplements.
There are a few situations that may require extra calcium. Mothers under 18 (their own bones are still growing), multiple fetuses (obviously the fetal requirements are multiplied), and heparin use (not common).
So take a deep breath, relax, and enjoy some nourishing calcium-containing foods, but don't stress. Your body knows what it's doing. If you want to geek out and read the summary from the Journal of Perinatal Education, go ahead! It contains all the study information I used.