Normal and Abnormal Lab Values in Pregnancy

Introduction

• During pregnancy, there are many physiologic changes, including increases in cardiac output, blood volume, glomerular filtration rate, and metabolism. There are a variety of changes in coagulation factors leading to a hypercoagulable state.
• These changes are reflected in lab values, so “abnormal” values flagged by the lab may not reflect normal values for pregnancy. On the other hand, lab values may not be flagged as they are normal for the non-pregnant state, but they are, in fact, abnormal for pregnancy.

Hematologic Changes¹

Hematologic

• Plasma volume increases by about 50% during pregnancy, with the majority of the increase happening between 32 and 34 weeks of gestation.
• Hemoglobin, hematocrit, and red blood cell (RBC) count decrease because plasma volume expands faster than the RBC cell mass increases. Mean corpuscular volume and mean corpuscular hemoglobin concentration are usually unaffected by hemodilution.
  • Platelet counts are usually unaffected unless there’s an underlying pathology such as gestational thrombocytopenia.
• Iron demand increases 2-3-fold, folate increases 10-20-fold, and B12 demand increases 2-fold.
• Clotting factors VIII, IX, X, and fibrinogen increase dramatically while fibrinolytic activity decreases.
  • Notably, a seemingly normal fibrinogen level by lab values may be too low for a pregnant patient.
• There are lower amounts of endogenous anticoagulants, including proteins S and antithrombin.¹
  • Thus, pregnant and postpartum women are in a physiologic hypercoagulable state.¹

Respiratory Changes

Respiratory

• Oxygen demand increases by about 20% and minute ventilation increases by 40-50%.
• On blood gas, arterial PO₂ increases and PCO₂ decreases. As a compensatory measure, serum bicarbonate decreases (from renal excretion) to 18-22 mmol/L, demonstrating a compensated respiratory alkalosis.

Metabolic Changes¹

Thyroid

• T4 and T3 have a moderate change, but with supplemental iodine, they should not have a clinically significant change unless there’s an underlying thyroid pathology.

Renal and Adrenal

• The renin-angiotensin-aldosterone system is activated by reduced SVR, leading to a threefold increase in aldosterone levels in the first trimester and a tenfold increase in the third.
  • Adrenocorticotropic hormone, cortisol, free cortisol, deoxycorticosterone, and corticosteroid-binding globulin all rise during pregnancy, leading to typical signs of hypercortisolism such as striae, facial plethora, and impaired glucose tolerance.
  • The placenta releases corticotropin-releasing hormone, which also constricts to hypercortisolism during late pregnancy.

Pituitary

• The pituitary gland grows as prolactin-producing cells increase, with prolactin levels tenfold higher during the first term.

Metabolic

• Maternal insulin resistance begins to develop during the second trimester and peaks during the third trimester.
• Fasting glucose levels are reduced due to:
  • Increased peripheral glucose use
  • Increased storage of tissue glycogen
  • Uptake of glucose by the fetus
• The placenta produces aminopeptidase vasopressinase, which increases the metabolic clearance of vasopressin, and a temporary diabetes insipidus may occur.
  • Atrial natriuretic peptide secretion increases by 40% as a result of this volume expansion.
  • Pregnant women with pre-eclampsia and persistent hypertension have a larger amount of natriuretic peptides.
• Total serum cholesterol and triglyceride levels rise during pregnancy to promote fatty acid metabolism in the mother, thereby allowing higher glucose levels and increased glucose uptake by the fetus.
• Although there is increased calcium uptake and requirement during pregnancy, total serum calcium concentration decreases due to hemodilution, which lowers serum albumin levels and the albumin-bound portion of calcium.
  • There is a subsequent increase in parathyroid hormone to allow for increased calcium release if calcium intake is insufficient.
• Recent research has demonstrated how significantly lab values such as folic acid, C-reactive protein, and albumin levels are affected by healthy behaviors.²

Normal Lab Values in Pregnancy

Hematologic and Biochemical Reference Values During Pregnancy³

Complete Blood Count

Coagulation Studies

Chemistry