Pregnancy Laboratory Test
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p429-430
2025-11-25
18
During pregnancy, it is important to carry out periodic laboratory tests, at least every 3 months, in order to evaluate the evolution of the pregnancy and to identify at an early stage any pathological alterations associated with the state of pregnancy and potentially dangerous to the health of the mother and the fetus, such as anemia or gestational diabetes; timely intervention through the administration of appropriate therapy can prevent complications. It is important to emphasize that pregnancy is a clinical condition characterized by physiological changes associated with altered levels of some laboratory parameters. It is therefore essential to know the reference values in pregnancy to correctly interpret the laboratory data.
Laboratory tests that should be performed during pregnancy are listed in Table 1.
Table1. Tests to be performed during pregnancy
Pregnancy is associated with physiological anemia, characterized by hemoglobin levels lower than the reference values due to hemodilution. From the sixth week of pregnancy, it is generally observed a rapid and important increase in plasma volume and, to a lesser extent, an increase in the volume of red blood cells due to the erythroid hyperplasia that characterizes pregnancy (the bone marrow meets the oxygen demand that increases progressively with the production of red blood cells). These physiological changes lead to a dilution of the blood, resulting in a reduction in hematocrit levels, red blood cells, and hemoglobin. The fall in hemoglobin concentration reduces blood viscosity, thus improving placental perfusion and facilitating the maternal–fetal exchange of oxygen and nutrients.
A slightly increased plasma volume with an increased hemoglobin concentration increases blood viscosity, reducing blood flow to tissues (possible preeclampsia). In pregnancy, a hemoglobin value ≥11 g/dL is considered normal; hemoglobin values ≥11 g/dL is considered normal; hemoglobin values <11 g/dL should raise the suspicion of pathological anemia and, therefore, require further investigation. Ferritin is the most sensitive and specific biomarker to assess the extent of iron stores and establish the sideropenic nature of anemia. Hemoglobin values <8.5 g/dL are associated with an increased risk of poor neonatal outcomes. Plasma transferrin increases by 1.5–2 fold. In the last trimester of pregnancy, 90% of plasma iron is released via transferrin to receptors located on the membrane of syncytiotrophoblast cells, which are the same as those found on the membranes of reticulocytes. Therefore, ferritin decreases during pregnancy, partly due to hemodilution and partly due to the depletion of iron resources.
At the end of the third trimester of pregnancy, mild thrombocytopenia (with platelet counts not less than 100,000/mL) can be observed, which is generally a benign, asymptomatic condition that tends to resolve within 2 months of delivery. There may also be a slight increase in white blood cells, pre dominantly neutrophils, due to the action of estrogen. In general, pregnancy is characterized by a state of physiological hypercoagulability to ensure the deposition of fibrin between the uterine wall and the chorionic villi and maintain placental integrity; it is observed, therefore, an increase in fibrinogen, factors V, VII, VIII, IX, and X, while decreasing fibrinolytic activity, free protein S, and antithrombin III. This condition determines an increased thrombotic risk from 4 to 10 times throughout the pregnancy that remains until 2–3 weeks after delivery. In addition to the proteins involved in the hemostatic process, pregnancy is characterized by alterations in the serum concentration of other proteins produced in the liver. In particular, there is an increase in alkaline phosphatase, in part due to the placental production of one of its isoenzymes, a reduction in gammaglutamylpeptidase, and serum albumin. Additionally, total proteins and the albumin/globulin ratio are significantly reduced, while aspartate aminotransferase and alanine transaminase remain unchanged.
Pregnancy also induces adaptations in renal function to allow the gain of Na+ and water to ensure the expansion of extracellular volume and the growth of the fetus. During pregnancy, glomerular plasma flow increases progressively, reaching a maximum peak around the 20th week of gestation (up to 50% more) and remaining at this level throughout the pregnancy. In parallel, the glomerular filtration rate increases. Under these conditions, there is an increase in glucose excretion (sometimes glycosuria can be observed), creatinine, urea, proteins, and uric acid in the first trimester. Circulating glucose, creatinine, urea, and protein tend to decrease during pregnancy; uric acid is low in the first trimester, increases in the second, and is higher than normal in the third trimester. This occurs because tubular reabsorption changes normalize a few weeks after delivery.
The urine test allows to detect bacterial infections of the urinary tract, which are quite common in pregnancy, or other changes that may pose a risk to the fetus and the mother.
The presence of bacteria can be associated with the typical symptoms of cystitis (burning during urination and frequent urination) or be asymptomatic. Early detection of bacteriuria is important because it is associated with an increased risk of pyelonephritis due to the increased uterus volume, which compresses the bladder and urinary tract and causes stagnation of urine. In this condition, bacteria in the urine are more likely to travel up to the kidneys and cause the infection. In addition, untreated bacteriuria is associated with an increased risk of low birth weight and preterm birth due to premature rupture of membranes. Urinary tract infections in pregnancy could also trigger situations that lead to gestational hypertension or preeclampsia. The bacterium responsible for most infections is Escherichia coli.
The test is performed on a urine sample collected in the morning (intermediate micturition). The presence of nitrites, significant bacteriuria (> 100,000 colonies per mL of urine), and increased leukocytes should raise the suspicion of urinary tract infection; to confirm the suspicion, a urinalysis with an antibiogram should be performed to isolate the specific pathogenic microorganism, determine antibiotic sensitivity, and direct the choice of therapy. The test is positive for a growth rate of ≥ 105 colony-forming units/mL. A urine culture is also performed in the presence of typical symptoms commonly referred to as cystitis.
Squamous epithelial cells derived from the vaginal epithelium can be found in urine. If present in low concentrations and in the absence of other altered parameters, the data is irrelevant, but if present in high concentrations and associated with bacteriuria and blood counts, it could indicate the presence of vaginitis. In this case, a vaginal swab should be performed to confirm the presence of an infection and proceed with antibiotic therapy.
Erythrocytes should not be detectable in urine. However, their presence in the urine sediment could indicate cystitis, a cervical polyp, a vaginal fold, or inflammation of the lower urinary tract; it could also be due to recent intense sexual activity.
Glycosuria should not be present. However, small amounts of glucose in the urine may be found in pregnancy due to physiological changes in the kidney; high concentrations may indicate gestational diabetes.
Protein should generally not be detectable in the urine, although a certain amount of protein is physiologically lost through the urine. Detecting proteinuria before the 20th week of gestation may indicate a renal alteration, while a later finding may indicate preeclampsia or gestosis.
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