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.
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.
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.
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.
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.
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