The Fetal Chest

Pulmonary Development

In the human lung, there are five distinct stages of development, during which the lung matures and the number of alveoli increases. At birth, the lungs are functional but structurally immature; the greatest increase in number of alveoli occurs postnatally. During the first 3 years of life, the alveoli are formed through a septation process that increases the gas exchange surface area. It is important to understand that this lung development process is ongoing, and that space-occupying lesions, or extrinsic abnormalities that do not allow for normal lung growth, can lead to improper lung development.

Normal Sonographic Features of the Fetal Chest

The fetal lungs are identified by ultrasound as homogeneously echogenic tissue surrounding the heart, separated by the hypoechoic, dome-shaped diaphragm from the abdominal organs ( Fig. 36.1A and B ). The fetal ribs are highly echogenic, curved bony structures arising near the spine and extending anteriorly to encompass more than half the thoracic circumference. Lung echogenicity varies during gestation, in general increasing in echogenicity as lung development progresses.

Assessment of lung volume is important for evaluation of fetuses at risk for pulmonary hypoplasia, particularly in cases of congenital diaphragmatic hernia (CDH), pleural effusions, prolonged oligohydramnios, and skeletal deformities. Methods to assess pulmonary size include measurement of the thoracic circumference, lung area (defined as internal thoracic area minus cardiac area in diastole on a transverse four-chamber view ), and three-dimensional lung volumetry with ultrasound or MRI.

The fetal lungs, thorax, and heart grow at similar rates, such that the normal cardiothoracic ratio remains constant in the second and third trimesters. On a normal four-chamber transverse view of the heart, the heart should occupy one-third to one-half the sonographic diameter of the thorax.

The cardiac position and axis are constant in normal fetuses. The apex of the heart points left and touches the anterior chest wall. The posterior aspect of the right atrium lies to the right of midline ( Fig. 36.1E ).

Familiarity with the normal anatomy and position of the fetal heart is crucial, along with in utero establishment of the right and left sides of the fetus. The reference to cardiac position and situs is identified by noting that the left atrium lies posteriorly, closest to the spine, and the right ventricle lies anteriorly, closest to the chest wall. Any deviation in the position of the heart should prompt a search for cardiac or pulmonary abnormalities. Anatomy of the fetal heart, including size and position, can be easily influenced by extracardiac thoracic anomalies.

Normal Diaphragm

Early in embryogenesis, the narrow pleuroperitoneal duct connects the pleural and peritoneal cavities. The development of the diaphragm, at 9 weeks' gestation, divides the two cavities. The normal diaphragm can be visualized as early as 10 weeks of gestation. The diaphragm appears as a thin, hypoechoic, arched line separating the chest from intraabdominal contents ( Fig. 36.1C-F ). It is best recognized as a dome on each side on sagittal and coronal views, with no difference in the height of the diaphragm on either side. The intact left hemidiaphragm is more easily verified by the presence of the fluid-filled stomach in the abdomen. On the right side, however, meticulous effort is required to identify the hypoechoic linear muscular diaphragm between the liver and lung.

Normal Thymus

The fetal thymus can be identified as early as 14 weeks' gestation in the anterior mediastinum. By the third trimester, the thymus is visualized as an ovoid, relatively hypoechoic structure ( Fig. 36.1G and I ). The thymus contains spindle-shaped echogenicities that differentiate it from the surrounding echogenic lungs. Thymus size varies greatly during gestation. Thymic imaging and measurements are not performed routinely on prenatal scans. However, prenatal identification and measurement of the fetal thymus is important when DiGeorge syndrome (with thymic aplasia or hypoplasia) is suspected. At times a large thymus will be confused with a mediastinal mass. Homogeneity and lack of mediastinal deviation can help differentiate normal thymus from a mediastinal teratoma or thymic cyst. The normal thymic average transverse measurement is 12 mm at 19 weeks' gestation and 33 mm at 33 weeks. The normal thymic average perimeter is 128 mm at 38 weeks. Acute fetal thymic involution has been reported in association with chorioamnionitis.

Mediastinal teratomas are rare anterior mediastinal complex masses. By ultrasound, the mass is heterogeneous and calcifications may be noted. MRI is a useful adjunct in the assessment of the mass for potential in utero and postnatal surgical treatment. The mass can cause polyhydramnios secondary to esophageal compression, and hydrops due to impaired venous return with resultant fetal demise. Fetal intervention with aspiration of the tumor cyst has been described with a resultant decrease in intrathoracic pressure, which may stop the progression to hydrops and prevent lung hypoplasia. The ex utero intrapartum treatment (EXIT) procedure may be recommended to establish a proper airway at delivery.