Calcium

Many people are deficient in calcium, putting them at risk for stress fractures, osteoporosis and other bone diseases.  If you don't get enough dairy in your diet, you may need a supplement, especially in pregnancy.  But, there are other foods rich in calcium such as sardines, salmon with bones, collard greens, and products supplemented with calcium like some orange juices and breads.

How much calcium should pregnant women get?

If you are 19 years of age or older you need 1,000 mg calcium daily
If you are 18 or younger you need 1,300 mg calcium daily

I know a doctor who recommends one extra serving of calcium-rich foods (milk, yogurt, cheese, ice cream, pudding, etc.) daily for pregnant women to ensure good intake and absorption.

Supplements can help but are not absorbed as well as natural food sources so you may need more.  Your prenatal vitamin probably contains extra calcium but if you don't use dairy products you may need more.  Click here for more information on supplements, including the various types - citrate, carbonate, etc.  If taking 1,000 mg calcium, it's best to divide the dose into two 500 mg taken at different times of the day (morning, night with one of the doses being the prenatal vitamin) since absorption will be better that way.