First, it meets the metabolic demands of the enlarged uterus and its greatly hypertrophied vascular system. Second, it provides abundant nutrients and elements to support the rapidly growing placenta and fetus. Third, the expanded intravascular volume protects the mother, and in turn the fetus, against the deleterious efects of impaired venous return in the supine and erect positions. Last, it safeguards the mother against the adverse efects of parturition-associated blood loss.
Maternal blood volume begins to accrue during the irst trimester. By 12 menstrual weeks, plasma volume expands by approximately 15 percent compared with that prior to pregnancy (Bernstein, 2001). Maternal blood volume grows most rapidly during the midtrimester, rises at a much slower rate during the third trimester, and reaches a plateau during the last 6.5 � 6.0 ! 5.5 5.0 4.0 3.5 • Twin pregnancy• Singleton pregnancy 3.0 7 -6 -ls 5E 4)E) 3�'5r� 2:� Menses First Second Third Postpartum
FIGURE 4-6 Estimated daily iron requirements during pregnancy in a 55-kg woman. (Modified from Koenig, 2014.) (n 10) and singletons (n 40). Data shown as medians. (Data from Thomsen, 1994.) several weeks of pregnancy (Fig. 4-5). Blood volume accrues even more dramatically in twin gestations. During blood volume expansion, plasma volume and erythrocyte number rise. Although more plasma than erythrocytes is usually added to the maternal circulation, the increase in erythrocyte volume is considerable and averages 450 mL (Pritchard, 1960). Moderate erythroid hyperplasia develops in the bone marrow, and the reticulocyte count is elevated slightly during normal pregnancy. These changes are almost certainly related to an elevated maternal plasma erythropoietin level.
Because ofgreat plasma augmentation, both hemoglobin concentration and hematocrit decline slightly during pregnancy (Appendix, p. 1255).s aresult, whole bloodviscositydecreases (Huisman, 1987). Hemoglobin concentration at term averages 12.5 g/dL, and in approximately 5 percent ofwomen it is below 11.0 g/dL. Thus, a hemoglobin concentration below 11.0 g/dL, especially late in pregnancy, is considered abnormal and usually due to irondeficiency anemia rather than pregnancy hypervolemia.
The total iron content of normal adult women ranges from 2.0 to 2.5 g, or approximately half that found normally in men. Most of this is incorporated in hemoglobin or myoglobin, and thus, iron stores of normal young women only approximate 300 mg (Pritchard, 1964). Although the lower iron levels in women may be partly due to menstrual blood loss, other factors have a role, particularly hepcidin-a peptide hormone that functions as a homeostatic regulator of systemic iron metabolism. Hepcidin levels rise with inlammation, but drop with iron deficiency and several hormones, including testosterone, estrogen, vitamin D, and possibly prolactin (Uu, 2016; Wang, 2015). Lower hepcidin levels are associated with greater absorption ofiron via ferroportin in enterocytes (Camaschella, 2015).