Maternal Physiology: Hematologic System

Hematologic System

The physiologic adaptations in the maternal hemato-logic system maximize the oxygen-carrying capacity of the mother to enhance oxygen delivery to the fetus. In addition, they minimize the effects of impaired venous return and blood loss associated with labor and delivery.

ANATOMIC CHANGES

The primary anatomic adaptation of the maternal hematologic system is a marked increase in plasma volume, red cell volume, and coagulation factors. Maternal plasma volume begins toincrease as early as the sixth week of pregnancy and reaches a maximum at 30 to 34 weeks’ gestation, after which it sta-bilizes. The mean increase in plasma volume is approxi-mately 50% in singleton gestations and greater in multiple gestations. Red cell volume also increases during pregnancy, although to a lesser extent than plasma volume, averaging about 450 mL. Maternal blood volume increases 35% by term.

Adequate iron availability is essential to the increase in maternal red cell volume during pregnancy. The normal pregnant patient requires a total of 1000 mg of additional iron: 500 mg is used to increase maternal red cell mass, 300 mg is transported to the fetus, and 200 mg is used to compensate for normal iron loss. Because iron is actively transported to the fetus, fetal hemoglobin levels are main-tained regardless of maternal iron stores. Supplemental iron use in pregnancy is intended to prevent iron deficiency in the mother, not to prevent either iron deficiency in the fetus or to maintain maternal hemoglobin concentration.

To meet maternal iron needs in a woman who is not anemic, 60 mg of elemental iron is recommended daily.

Iron from dietary sources may not be sufficient, and the National Academy of Sciences recommends an iron supple-ment of 27 mg (present in most prenatal vitamins). In the form of ferrous sulfate, 60 mg of iron is a dosage of 300 mg. Patients who are anemic should receive 60–120 mg of iron. Leukocyte count and platelet counts may vary during preg-nancy. White blood cell counts typically increase slightly in pregnancy, returning to nonpregnant levels during the puerperium. During labor, the white blood cell count may further increase, primarily from increased granulocytes, pre-sumably linked with stress-associated demargination rather than a true disease-associated inflammatory response. Platelet counts may decline slightly, but remain within the normal, nonpregnant range.

The concentration of numerous clotting factors is increased during pregnancy. Fibrinogen (factor I) increases by 50%, as do fibrin split products and factors VII, VIII, IX, and X. Prothrombin (factor II) and factors V and XII remain unchanged. In contrast, the concentration of key inhibitors of coagulation, activated protein C and protein S, both decrease.

FUNCTIONAL CHANGES

During pregnancy, functional adaptations in maternal erythrocytes enable enhanced oxygen uptake in the lungs,

allowing increased oxygen delivery to the fetus and pro-moting CO₂ exchange from fetus to mother. The increasein oxygen delivery to the lungs and the amount of hemoglobin in the blood result in a significant increase in the total oxygen-carrying capacity. In addition, the compensated respiratoryalkalosis of pregnancy causes a shift in the maternal oxygen dissociation curve to the left, via the Bohr effect. In the maternal lungs, hemoglobin affinity for oxygen increases, whereas in the placenta, the CO₂ gradient between fetus and mother is increased, which facilitates transfer of CO₂ from fetus to mother. See p. 54 for further discussion.

Pregnancy is considered a hypercoagulable state with an increased risk of venous thromboembolism, both during preg-nancy and the puerperium.

The risk of thromboembolism doubles during pregnancy and increases to 5.5 times the normal risk during the puer-perium.

SYMPTOMS AND PHYSICAL FINDINGS

Some edema is normal in pregnancy, and swelling of the hands, face, legs, ankles, and feet may occur. This tends to be worse late in pregnancy and during the summer.

DIAGNOSTIC TESTS

Pregnancy results in alterations in the normal ranges of sev-eral hematologic indices. The disproportionate increase in plasma volume, compared with red cell volume, results in a decrease in hemoglobin concentration and hemat-ocrit during pregnancy, often referred to as a physiologicanemia. At term, the average hemoglobin concentration is12.5 g/dL, compared with approximately 14 g/dL in the nonpregnant state. Values less than 11.0 g/dL are usually due to iron deficiency, but such values should prompt inves-tigation for other kinds of anemia that may occur simulta-neously with iron-deficiency anemia. Treatment of any anemia should be administered. The leukocyte count can range from 5000 to 12,000/L, and may increase to as much as 30,000/L during labor and the puerperium. (Neither of these higher values is associated with infection.)

The most notable alteration in the coagulation system is increased concentration of fibrinogen, which ranges from 300 to 600 mg/dL in pregnancy, compared with 200 to 400 mg/dL in the nonpregnant state. Despite the prothrombotic state of pregnancy, in vitro clotting times do not change.