![]() Red cell mass (driven by an increase in maternal erythropoietin production) also increases, but relatively less, compared with the increase in plasma volume, the net result being a dip in hemoglobin concentration. This suggests that, in pregnant state, the elevation in plasma volume is in response to an underfilled vascular system resulting from systemic vasodilatation and increase in vascular capacitance, rather than actual blood volume expansion, which would produce the opposite hormonal profile instead (i.e., low plasma renin and elevated atrial natriuretic peptide levels). During pregnancy, plasma renin activity tends to increase and atrial natriuretic peptide levels tend to reduce, though slightly. ![]() Expansion of plasma volume occurs by 10–15 % at 6–12 weeks of gestation. ![]() Increase in blood volume is, therefore, more marked in multiple pregnancies and in iron deficient states. Of this, around one liter of blood is contained within the uterus and maternal blood spaces of the placenta. During pregnancy, the total blood volume increases by about 1.5 liters, mainly to supply the demands of the new vascular bed and to compensate for blood loss occurring at delivery.
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