Introduction

Hydrops fetalis (HF) literally means “an abundance of water”; hydrops is similar to “the dropsy” from which patients with congestive cardiac failure used to suffer. HF fetuses have accumulations of fluid in pleural, pericardial, and peritoneal cavities, as well as subcutaneously. The majority of cases of HF are detected by antenatal ultrasound. There are many individual diseases that result in HF, and these disorders can be categorized in five main groups (Figure 5.1):

1. (1) cardiac failure (structural and functional)

2. (2) lymphatic system obstruction/malformation (dysplasia)

3. (3) thoracic cavity space occupation with increased pressure

4. (4) abdominal cavity space occupation with increased pressure

5. (5) fetal hypokinesia.

Underlying these categories may be chromosome abnormalities, single gene disorders, and infections. Currently, Online Mendelian Inheritance in Man lists 116 mutated single genes and their corresponding single gene disorders that are associated with HF; Google Scholar currently lists almost 17 000 published papers on hydrops fetalis, many of which were written in 1980–2010. At first the interlocking causes of HF may seem overwhelmingly complex, but placing them in the main groups described above and establishing chains of causation can simplify them. Major blood group alloimmunization was previously a frequent cause of HF in populations with, for instance, a prevalence of Rhesus D antigen incompatibility, but this is no longer the case, and other causes of (non-immunologic) HF are much more common. Primary fetal hypoproteinemia is not involved widely in HF; for instance, Finnish nephrosis and analbuminemia rarely, if ever, present clinically with HF. It is not always obvious how an underlying metabolic genetic disease (e.g., a storage disorder) causes HF, but hepatic involvement may result in reduced albumin synthesis (see below) and there may also be cardiomyopathy. In addition, large amounts of plasma proteins are sequestered in the third space of HF cases.