Multiple physiologic changes occur during pregnancy.  A decrease in systemic vascular resistance is the primary reason for a decrease in blood pressure during pregnancy.  Increases in cardiac output often cause an increase in blood pressure.  An increased afterload also increases systemic arterial blood pressure.  While a decrease in hemoglobin (or anemia) may cause blood pressure to decrease, the anemia of pregnancy is actually caused by a greater increase in plasma volume than in red cell volume.  This does not cause the decrease in blood pressure noted during mid-pregnancy.