Pregnancy in the human female is a unique state in which virtually all maternal systems are dramatically altered to permit the sustenance and growth of the intrauterine conceptus. Major physiological changes include cardiovascular, hematologic, metabolic, renal, and respiratory changes, most of which begin soon after conception and continue throughout pregnancy till until late gestation. These changes affect various patient laboratory test results. The body can generally compensate for these changes . However, in the presence of conditions such as anemia, clotting disorders, bleeding during pregnancy, preeclampsia, and trauma caused by motor vehicle accident, the body may not be able to compensate for the changes. At this point, laboratory values can become significantly skewed from the values normally noted during pregnancy. In caring for pregnant women and their unborn infants, it is important for the healthcare provider to understand the normal physiologic changes that occur during pregnancy. The provider can utilize various laboratory tests and diagnostic tools to assess the magnitude of these changes and to identify abnormal changes. It is imperative that they should be aware of both the normal and abnormal laboratory values to be able to make decisions about clinical management of the woman. However, very few laboratories provide clinicians with normal reference ranges during pregnancy. This chapter makes an attempt to discuss the physiological changes and alterations in the laboratory values that occur during pregnancy.