Explain Expansion in Plasma Volume During pregnancy?

In a non-pregnant woman, average plasma volume is 2600 ml. Plasma volume begins to increase by end of the first trimester and reaches a peak by 34 weeks, when it is about 50% greater than it was at conception. The physiologic dilution accompanying the expansion in blood volume is known as haemodilution. Haemodilution is responsible for the decline we see in several blood parameters  during pregnancy especially in the second and third trimesters.

Individual variations are seen in plasma volume - multigravidas and in mothers with multiple births, usually have a greater increase. Increase in plasma volume is correlated with obstetric performance. Women with an increase smaller than the average have higher risk of still births, abortions or low birth weight babies. This has been seen in women with hypertension, renal disease, low weight gain during pregnancy, under nutrition, smoking, pre-eclampsia, diuretic treatment and smoking.