Blood test results during normal pregnancy and pregnancy-related illness

Pregnancy is associated with significant anatomical and physiological change in order to both accommodate and sustain the developing fetus. As this literature review article written by two Australian obstetricians makes clear, pregnancy-associated physiological change affects the blood concentration of a range of biochemical and hematological parameters routinely measured in the laboratory or at the point of care.

The intended audience of this article is non-obstetric clinical staff who have to assess pregnant women presenting at the emergency room. The stated dual aim is to first outline the quite normal pregnancy-associated changes in laboratory test values, and then to discuss the common causes of deviations from these normal ( pregnancy-associated) test values, with a particular focus on pregnancy-specific disorders that might provoke referral to emergency care. 

The article is presented under the following 11 test headings: cardiac biomarkers (troponin BNP); arterial blood gases; sodium/osmolality; potassium; renal (urea creatinine eGFR); liver function tests; calcium/magnesium; thyroid; hypothalamic-pituitary-adrenal axis; full blood count; and finally, coagulation tests.

Under each of these headings the authors briefly describe the pregnancy-related physiological change that causes measured parameter to be different from normal (non-pregnant) healthy status, and then go on to discuss the relevance of this difference for clinical assessment of pregnant women, and where appropriate, how the test result is affected by pregnancy-specific disorders. 

So, for example, under the heading arterial blood gases, we learn that normal pregnancy is associated with progesterone-mediated increase in minute ventilation that results in increase in blood oxygenation (i.e. increased pO₂(a)) and decrease in carbon dioxide (reduced pCO₂(a)). Reduction in pCO₂(a) results in respiratory alkalosis that is fully compensated for by increased renal excretion of bicarbonate.

In summary, normal pregnancy is associated with increase in pO₂(a), decreased pCO₂(a) and bicarbonate, but pH remains essentially unchanged. The authors caution that the finding of pCO₂(a) within the non-pregnant reference range may thus represent respiratory fatigue in a pregnant woman who is suffering acute exacerbation of asthma. 

As a consequence of the pregnancy-related reduction in bicarbonate (reduced buffering capacity), pregnant women with diabetes may develop diabetic ketoacidosis more rapidly, and at lower blood glucose concentration than non-pregnant diabetics. For the same reason, pregnant women are more susceptible to starvation ketoacidosis than non-pregnant individuals. 

Under the heading sodium/osmolality, the authors report that normal pregnancy is associated with reduction in the osmotic threshold that causes release of the antidiuretic hormone, arginine vasopressin (AVP) and invokes the thirst response.

As a consequence of this physiological adaptation, normal pregnancy is associated with reduction in serum sodium of 3-6 mmol/L and reduction in serum osmolality of 10 mOsm/kg. Hyponatremia is diagnosed if serum sodium <135 mmol/L in non-pregnant individuals, but <130 mmol/L in pregnant women. 

Preeclampsia is, the authors report, the most common cause of hyponatremia during pregnancy, occurring in 9.7 % of preeclampsia-affected pregnancies according to one study they cite. Detail of the mechanism of this preeclampsia-related hyponatremia is discussed. Other less common causes of pregnancy-related hyponatremia include: prolonged labor; post Cesarean section; oxytocin infusion; primary polydipsia (compulsive water drinking); and hyperemesis gravidarum (extreme form of morning sickness). 

Under the renal heading, the authors remind of the well-documented pregnancy-associated increase in renal plasma flow (RPF) and glomerular filtration rate (GFR); both rise by approximately 60 % early in the first trimester, before declining to normal non-pregnant values during the third trimester. As a consequence of reduced RPF/GFR, serum urea, creatinine and urate concentrations are all reduced by 30 % during healthy pregnancy. 

Normal pregnancy is associated with slight rise in urine excretion of both total protein and albumin but more significant increase may signal either pre-conception proteinuria or preeclampsia. The authors report that preeclampsia is the most common cause of acute kidney injury (AKI) during pregnancy (accounting for 60-80 % of such cases in the developed world). Other causes of pregnancy-related AKI include: placental abruption, sepsis, postpartum hemorrhage, acute fatty liver of pregnancy (ALFP), and thrombotic thrombocytopenic purpura-hemolytic-uremic syndrome (TTP-HUS). 

This article, which lends on 148 cited references, provides a valuable up-to-date overview of not only the effect normal pregnancy has on a comprehensive range of laboratory blood tests, but also how laboratory testing can be used in the assessment of pregnant women presenting to the emergency department with a possible range of pregnancy-related disorders. The authors helpfully include a list of reference intervals for most of the laboratory analytes discussed; this list includes reference intervals for pre-pregnancy and for each trimester of pregnancy.