Version May 2024
INTRODUCTION — Numerous physiologic 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 physiologic or dilutional anemia of pregnancy. Similarly, the kidney 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 female is immediately recognized as elevated in pregnancy.
Despite the well-recognized phenomenon of pregnancy-induced physiologic changes and their potential for altering laboratory values, very few laboratories provide clinicians with reference intervals during pregnancy. Indeed, many laboratories do not even report reference intervals for females versus males. This topic will discuss reference intervals (or ranges) for laboratory values during pregnancy based upon the author's review of the literature [1-75].
REFERENCE INTERVALS IN PREGNANCY — A pregnancy laboratory reference interval is an approximation of what can be expected in the overall healthy pregnant population [76]. It does not necessarily indicate the presence or absence of a disorder. For example, a result within the reference interval does not necessarily exclude the presence of a disorder in a specific patient and a result outside of the interval does not necessarily indicate the presence of a disorder in a specific patient. For example, the lower ferritin value in the pregnancy reference interval is 5 ng/mL, but clinicians consider a ferritin <30 ng/mL sufficient to diagnose iron deficiency.
Previous investigators have compiled information on laboratory reference intervals in pregnancy [30,40,43,77]. Using these references, as well as publications by other researchers in which normal intervals were determined across pregnancy for a number of analytes, the author of this topic has compiled a table of the most common, and some not so common, laboratory test reference intervals across pregnancy (table 1) [1-75].
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 pregnancy. Similarly, the upper reference interval limit for D-dimer nearly doubles during midpregnancy. Several hormones and coagulation factors all increase markedly. Unless these physiologically induced pregnancy-related alterations are taken into account when evaluating laboratory values in a pregnant patient, many of the physiologic adaptations of pregnancy can be misinterpreted as pathologic or may mask diagnosis of a disease process.
The author of this topic considers these data the best available information on reference intervals in pregnancy; however, there are some limitations. The analysis is subject to the inherent limitations of abridged data and does not account for potential variations, such as time of day of sampling. Although he has 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. Despite these limitations, the table is intended to provide a quick reference for most laboratory values needed to provide care for the pregnant patient.
ADDITIONAL RESOURCES — Additional information on physiologic changes during pregnancy can be found in the following UpToDate topic reviews:
●(See "Maternal adaptations to pregnancy: Hematologic changes".)
●(See "Maternal adaptations to pregnancy: Cardiovascular and hemodynamic changes".)
●(See "Maternal adaptations to pregnancy: Gastrointestinal tract".)
●(See "Maternal adaptations to pregnancy: Musculoskeletal changes and pain".)
●(See "Maternal adaptations to pregnancy: Skin and related structures".)
●(See "Maternal adaptations to pregnancy: Renal and urinary tract physiology".)
●(See "Clinical manifestations and diagnosis of early pregnancy".)
●(See "Immunology of the maternal-fetal interface".)
SUMMARY AND RECOMMENDATIONS
●Laboratory test values across pregnancy – Numerous physiologic changes occur during pregnancy to accommodate the maternal and fetal needs. This table shows common, and not so common, laboratory reference intervals across pregnancy (table 1). (See 'Reference intervals in pregnancy' above.)
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