Chapter 57: Amniocentesis

INTRODUCTION

Amniocentesis is the procedure by which amniotic fluid is removed from the uterine cavity via a transabdominal approach (Fig. 57-1). While the vast majority of amniocenteses are performed in the outpatient setting by perinatologists in the previable period, there are indications for amniocentesis in the inpatient setting as well. Therefore the ability to perform an amniocentesis is a very useful tool in the obstetric and gynecologic (OB/GYN) hospitalist's arsenal. This chapter focuses on reviewing the critical aspects of the amniocentesis procedure as it is performed for inpatient indications.

PHYSIOLOGY

Amniotic fluid is an incredibly complex substance; the origins, composition, properties, and functions of which evolve dramatically over the course of the pregnancy. Prior to fetal skin keratinization, which occurs gradually between approximately 19 and 25 weeks gestation, amniotic fluid composition closely mirrors fetal plasma composition due to relatively free diffusion between the amniotic and fetal extravascular compartments. In the second half of pregnancy, amniotic fluid is produced largely by fetal urination, accounting for 75% to 80% of amniotic fluid volume, with the remainder contributed by secretions of oral, nasal, tracheal, and pulmonary fluid.¹ The tests on amniotic fluid discussed in this chapter assess the properties of the secretions and sloughed cells from these compartments.

INDICATIONS

Overall, the most common purpose for amniocentesis by far is to obtain cells for fetal karyotype or genetic testing. As the implications and interpretation of these types of results are complicated and carry significant potential medicolegal complications, these tests generally should not be ordered unless the patient has undergone genetic counseling, effectively limiting such testing to the outpatient realm. Similarly, testing for evidence of fetal TORCH infection typically is performed only after consultation with a perinatologist and based on specific, concerning fetal findings on ultrasound. However, if a patient who has received counseling and declined amniocentesis later develops an additional indication for the procedure and elects to proceed, these studies may be ordered by an OB/GYN hospitalist in addition to the tests needed for the primary indication.

Amniocentesis procedures are generally divided into diagnostic and therapeutic categories. A diagnostic amniocentesis is performed to obtain amniotic fluid on which to perform diagnostic tests. A therapeutic amniocentesis is performed in order to relieve signs or symptoms caused by amniotic fluid volume and pressure. Inpatient indications that might reasonably arise within the scope of practice of an OB/GYN hospitalist are listed ...