Numerous physiological changes occur during pregnancy to accommodate the maternal and fetal needs. Most of these changes begin soon after conception and continue until late gestation. Not surprisingly, these physiologic adaptations of pregnancy result in many significant changes in laboratory test values. Some of these changes are well-known, such as the reduction in hematocrit and hemoglobin levels, which is termed physiological or dilutional anemia of pregnancy. Similarly, the renal changes leading to lower creatinine values in pregnancy are well-described and a "normal" serum creatinine value of 1.0 mg/dL in a nonpregnant woman is immediately recognized as elevated in the pregnant woman.
Despite the well-recognized phenomenon of pregnancy-induced physiological changes and their potential for altering normal laboratory values, very few laboratories provide clinicians with normal reference ranges during pregnancy. Indeed, many laboratories do not even report normal values for women versus men. This topic will discuss normal reference ranges for laboratory values during pregnancy based upon our review of the literature [1-74].
NORMAL REFERENCE RANGES IN PREGNANCY
Previous investigators have compiled information on normal laboratory reference ranges in pregnancy [30,40,43,75]. Using these references, as well as publications by other researchers in which normal values were determined across pregnancy for a number of analytes, we compiled a table of the most common, and some not so common, laboratory test values across pregnancy (table 1) [2-74].
Laboratory test values are grouped by system and listed for each trimester and for nonpregnant adults. The table shows that some analytes, such as the leukocyte count and alkaline phosphatase levels, continue to rise during normal pregnancy. Similarly, the upper limit of normal for D-dimer nearly doubles from the first to the third trimester. Several hormones and coagulation factors all increase markedly. Unless these normal, pregnancy-related alterations are taken into account when evaluating laboratory values in a pregnant woman, many of the physiologic adaptations of pregnancy can be misinterpreted as pathologic or may mask diagnosis of a disease process.
We consider these data the best available information on normal reference ranges in pregnancy; however, with some limitations. The analysis is subject to the inherent limitations of abridged data, and does not account for potential variations between racial groups, regions of the world, and time of day of sampling. Although we have tried to include comparisons of analytes performed by similar analytic methods, some variation is inevitable. In the majority of instances, reference ranges for analytes are expressed as 5th to 95th percentiles, but some are reported with standard deviations. In addition, pregnancy-specific information is not always available for laboratory tests that have been newly introduced into medical care or have recently gained in popularity. For example, there is sparse information on some tests currently used in cardiac evaluation (atrial natriuretic peptide, B-type natriuretic peptide [BNP], troponin, creatine kinase, and creatine kinase-MB) in pregnancy. Despite these limitations, the table is intended to provide a quick reference for most laboratory values needed to provide care for the pregnant woman.