18.3 Fetal Infection, Neoplasms & Hydrops

TORCH Infections

TORCH is the mnemonic for the classic congenital infections: Toxoplasmosis, Other (syphilis, parvovirus B19, varicella), Rubella, Cytomegalovirus, and Herpes simplex. All can cross the placenta, and their common sonographic threads are intrauterine growth restriction (IUGR), hepatosplenomegaly, echogenic bowel, ascites, and hydrops.

Cytomegalovirus (CMV) is the most common congenital infection. Its signature is periventricular calcifications — bright echogenic deposits ringing the ventricles — reflecting the virus's affinity for the germinal matrix. Other findings include microcephaly, ventriculomegaly, periventricular echogenicity, echogenic bowel, hepatosplenomegaly, and IUGR. Toxoplasmosis also calcifies the brain, but its calcifications are diffusely scattered through the parenchyma rather than periventricular, and it produces hydrocephalus and chorioretinitis. Rubella, now rare where vaccination is routine, causes cardiac defects, cataracts, and sensorineural deafness.

Parvovirus B19 deserves special attention because it infects erythroid precursor cells, arresting red-cell production. The resulting severe anemia causes high-output cardiac failure and hydrops fetalis, often with a thickened placenta and cardiomegaly. The key monitoring tool is middle cerebral artery (MCA) Doppler: anemic blood is less viscous and flows faster, so an MCA peak systolic velocity above 1.5 multiples of the median (MoM) for gestational age predicts moderate-to-severe anemia noninvasively. This measurement has largely replaced routine cordocentesis for surveillance; confirmed severe anemia is treated with intrauterine transfusion.

Fetal Neoplasms

Sacrococcygeal teratoma (SCT) is the most common fetal neoplasm, arising from the coccyx at the base of the spine. It may be predominantly cystic or solid; the distinction matters because large solid, highly vascular tumors act as an arteriovenous shunt, stealing cardiac output and causing high-output cardiac failure, hepatomegaly, hydrops, placentomegaly, and polyhydramnios. Doppler shows high-velocity, low-resistance flow within the mass. Tumors are classified by how much extends externally versus internally into the pelvis, and dystocia or tumor rupture complicates delivery.

Cervical teratoma arises in the anterior neck, producing a large mixed solid-cystic mass that hyperextends the neck and can obstruct the airway — a reason for an ex utero intrapartum treatment (EXIT) delivery. Neuroblastoma is the most common fetal adrenal tumor, appearing as a suprarenal solid or cystic mass that must be distinguished from renal lesions; maternal catecholamine effects are occasionally seen. Congenital mesoblastic nephroma is the most common fetal and neonatal renal tumor (a solid intrarenal mass often with polyhydramnios) — note that despite the company it keeps, it is a mesenchymal tumor, not a blastoma. The true fetal blastomas are nephroblastoma (Wilms tumor), which is rare before birth, hepatoblastoma in the liver, neuroblastoma, and pleuropulmonary blastoma.

Hydrops Fetalis

Hydrops fetalis is the abnormal accumulation of fluid in at least two fetal body compartments. The qualifying findings are:

• Skin edema >= 5 mm thick, best seen over the scalp or chest wall
• Ascites — free fluid outlining the liver and bowel
• Pleural effusion — fluid surrounding the lungs
• Pericardial effusion — fluid around the heart

Supporting findings include placentomegaly (placental thickness greater than about 4-6 cm depending on gestational age) and polyhydramnios. Hydrops is a final common pathway, not a diagnosis, and the sonographer's job is to document it and hunt for the cause.

Immune Hydrops

Immune hydrops results from Rh alloimmunization: an Rh-negative mother carrying an Rh-positive fetus becomes sensitized to the D antigen — usually at a prior delivery — and her immunoglobulin G antibodies cross the placenta and destroy fetal red cells. The ensuing hemolytic anemia drives high-output failure and hydrops. It is now uncommon in screened populations because Rh immune globulin (RhoGAM), given at about 28 weeks and again postpartum to unsensitized Rh-negative mothers, prevents sensitization. Affected fetuses are followed with serial MCA peak systolic velocity measurements and transfused in utero when the value exceeds 1.5 MoM.

Non-Immune Hydrops

Non-immune hydrops fetalis (NIHF) now accounts for roughly 90% of cases. The differential is broad:

• Cardiovascular — structural heart disease and sustained tachyarrhythmias (the most common category)
• Chromosomal — Turner syndrome (cystic hygroma) and trisomy 21
• Hematologic — alpha-thalassemia major, fetomaternal hemorrhage
• Infectious — parvovirus B19 and cytomegalovirus
• Structural/tumors — sacrococcygeal teratoma, chest masses, diaphragmatic hernia
• Twin-twin transfusion syndrome in monochorionic twins
• Lymphatic dysplasias and inborn errors of metabolism

Monitoring

Once hydrops is identified, serial scans track the fluid collections, growth, amniotic fluid, and placental thickness. MCA Doppler screens for anemia as the cause, and technique matters: the vessel is sampled at its proximal third near its origin from the circle of Willis, with the angle of insonation kept as close to 0 degrees as possible, because the peak systolic velocity falls as the angle rises and an over-angled reading falsely lowers the value. Measurements are taken with the fetus at rest and converted to multiples of the median using gestational-age nomograms, with serial values trended rather than acted on in isolation. Umbilical venous Doppler and ductus venosus waveforms assess cardiac compromise; and the biophysical profile gauges overall fetal well-being. Finding two or more fluid compartments should always trigger a directed search of the heart, a check for arrhythmia, an MCA measurement, and a review of maternal blood type and antibody screen — the cause determines whether the condition is treatable in utero, as parvovirus anemia and Rh disease often are.