There are both subtle and substantial changes in hematological parameters during pregnancy and the puerperium, orchestrated by changes in the hormonal milieu. Red cell count and hematocrit (Hct) values are likewise lower in pregnancy, but the other red cell indices change little, although red cells show more variation in size and shape than in the non-pregnant state. There has been much discussion about the normal ranges for the different types of white blood cells (WBC). Lymphocyte count decreases during pregnancy through first and second trimesters, increases during the third trimester, but remains low in the early puerperium as compared to normal non-pregnant values. Screening tests used to assess the coagulation pathways include the activated partial thromboplastin time (APTT), the prothrombin time (PT), and the thrombin time (TT). There are changes in the balance of the natural anticoagulants during pregnancy and the puerperium.