Pregnant women who are at risk for developing postpartum depression will be enrolled in this study when they are between 16 to 26 weeks pregnant. All recruited women will undergo psychological testing for screening purposes. Women who are found to be depressed or suffering from other psychiatric disorders during screening will be referred to alternative treatment and will not be enrolled in this study. All participants will then be randomly assigned to receive either a calcium supplement (2 grams) or placebo on a daily basis. Treatments will continue throughout the remainder of each woman's pregnancy and for 12 weeks after she gives birth. Outcome measurements will include standardized questionnaires and psychiatric interviews to assess depression levels. If any participant exhibits significant depressive symptoms, she will be referred for psychiatric treatment. All measurements will be assessed at Weeks 26, 32 and 38 of the pregnancy, and 6 and 12 weeks after giving birth.