Introduction

The human hemostatic system involves a number of complex and highly regulated processes. In turn, these processes are interwoven with other important systemic and cellular systems (e.g., immune, hematologic, nutritional/gastrointestinal, and cardiovascular) and are thus sensitive to myriad biologic influences and events. During states of blood vessel injury the hemostatic system must be quick, localized, and reliable. Unfortunately, there are many inherited, acquired, and physiologic conditions that can disrupt the hemostatic system and lead to suboptimal or inappropriate clot formation. The chapter focuses on thrombophilias and nonphysiologic thrombi (thrombosis not required for hemostasis) as they relate to the pregnant woman.

Several national organizations, including the American College of Obstetricians and Gynecologists (ACOG) and the American College of Chest Physicians (ACCP) have published guidelines regarding thrombosis and pregnancy. These will be referred to throughout this chapter.

Hemostatic System

In the normally functioning hemostatic system, coagulation is mediated by a cascade of extracellular clotting factors and cellular components that seal breached endothelium with fibrin–platelet plugs. This process can be thought of as having four essential phases:

Effect of Pregnancy on Hemostasis

The normal physiology of pregnancy causes important changes in the hemostatic system, resulting in a hypercoagulable state. These changes include:

Although these changes may have evolved to protect the mother from excessive blood loss at delivery, they also increase the likelihood of pathologic thrombus formation.