Normal pregnancy is characterized by profound changes in almost every organ system to accommodate the demands of the fetoplacental unit. Hematological adaptation to the pregnant state will be reviewed here; the most significant hematological changes are (table 1):
- Physiologic anemia
- Mild thrombocytopenia
- Increased procoagulant factors
- Diminished fibrinolysis
Plasma volume increases by 10 to 15 percent at 6 to 12 weeks of gestation [1-3], expands rapidly until 30 to 34 weeks, after which there is only a modest rise (figure 1). The total gain at term averages 1100 to 1600 mL and results in a plasma volume of 4700 to 5200 mL, 30 to 50 percent above that found in nonpregnant women [1,4].
During pregnancy, plasma renin activity tends to be increased and atrial natriuretic peptide levels are slightly reduced. This suggests that the rise in plasma volume is in response to an underfilled vascular system caused by systemic vasodilatation and the rise in vascular capacitance. If expansion of blood volume was the initial event, renal and atrial volume sensors would respond, leading to the opposite hormonal profile (low plasma renin activity, elevated atrial natriuretic peptide) [5,6]. This hypothesis is also supported by the observation that increasing sodium intake does not lead to further volume expansion .
Postpartum, plasma volume decreases immediately after delivery, then increases again two to five days later, possibly because of a rise in aldosterone secretion, which occurs at this time. Plasma volume then decreases; it is still elevated by 10 to 15 percent above nonpregnant levels at three weeks postpartum, but is usually at normal nonpregnant levels at six weeks postpartum.