Pregnancy: laboratory measurements


Pregnancy is known to be a state of “physiologic anemia” due to the disproportionate increase in plasma volume relative to red blood cell volume. This increase in blood volume is necessary to supply the fetus and placenta and begins very early in pregnancy. The plasma volume is already increased by 10-15% at 6 weeks of gestation and increases to 30-50% greater than pre-pregnancy volume by term. The red cell volume, in contrast to this, is only increased by 20-30% at term. This increase is prompted by a higher level of erythropoietin. Thus, despite a higher red cell volume, the hematocrit will fall during pregnancy. The greatest time of disproportion between the plasma volume and red cell volume changes will be at 28-36 weeks. Anemia during pregnancy is defined differently than someone who is not pregnant. To be considered anemic during pregnancy the hemoglobin needs to be less than 11g/dL in the 1st or 3rd trimesters.

Other hematologic changes in pregnancy include an increase in white blood cells, particularly neutrophils, and a slight decrease in platelet count. The average white cell count during pregnancy is about 9-15k. It increases up to term, and can go as high as 25k during labor. Platelets, on the other hand, remain in the normal non-pregnant range but mean platelet counts may be slightly lower than in healthy non-pregnant women. The “low normal” range is considered to be about 106-120k.

There are also many changes in blood chemistries during pregnancy as well, mostly by the same dilutional mechanism as the hematocrit. Important among these are albumin, total protein and creatinine. Both albumin and total protein decrease by about 1g/dL by mid-pregnancy and creatinine decreases by about 0.3mg/dL. For other chemistry changes, see chart (below).

Test Nonpregnant Range Pregnancy Effect Gestational Timing
Blood Chemistry
Albumin 3.5-4.8 g/dL 1 g/dL decrease By midpregnancy
Calcium 9.0-10.3 mg/dL 10% decrease Falls gradually
Chloride 95-105 mEq/L No change  
Cholesterol 200-240 mg/dL 50% increase Rises steadily
Creatinine 0.6–1.1 mg/dL 0.3 mg/dL increase By midpregnancy
Fibrinogen 200–400 mg/dL 1.5-3x increase By term
Glucose, fasting 65–105 mg/dL 10% decrease Gradual fall
Potassium (plasma) 3.5–4.5 mEq/L 0.2–0.3 mEq/L decrease By midpregnancy
Protein (total) 6.5–8.5 g/dL 1 g/dL decrease By midpregnancy
Sodium 135–145 mEq/L 2–4 mEq/L decrease By midpregnancy
Urea nitrogen 12–30 mg/dL 50% decrease First trimester
Uric acid 3.5–8 mg/dL 33% decrease First trimester
Urine Chemistry
Creatinine 15–25 mg/kg/d No change  
Protein Up to 150 mg/d Up to 250–300 mg/day By midpregnancy
Creatinine clearance 90–130 mL/min/1.73 m2 40–50% increase By 16 weeks
Serum Enzymes
Alkaline phosphatase 30–120 U/L 2x to 4x increase By 20 weeks
Amylase 60–180 U/L Controversial  
Creatinine phosphokinase 26–140 U/L 2- to 4-fold increase After labor (MB bands as well)
Lipase 10–140 U/L No change  
Aspartate aminotransferase (AST) 5–35 mU/mL No change  
Alanine Aminotransferase (ALT) 5–35 mU/mL No change  
Formed Elements of Blood
Hematocrit 36–46% 4–7 point decrease Nadir at 30–34 weeks
Hemoglobin 12–16 g/dL 1.4–2.0 g/dL decrease Nadir at 30–34 weeks
Leukocyte count 4.8–10.8 x 103/mm3 3.5 x 103/mm3 increase Gradual increase to term, as high as 25 x 103/mm3 in labor
Platelets 150–400 x 103/mm3 Slight decrease Gradual


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See Also:

Pregnancy: Heme and electrolyte changes