The normal location of the placenta is near the fundus of the uterus along the anterior or posterior wall. In about 20% of cases one or several unicameral cysts or cyst-like spaces (64) appear within the placenta (94) (Fig. 66.1) and generally are inconsequential, though a certain association with maternal diabetes or rhesus incompatibility has been proposed. The placental location should not be definitively assessed before the end of the 2nd trimester since the placenta previa of an early pregnancy can become a normal or "low" lying placenta due to stretching of the uterine cervix (distance to the internal cervical os < 5 cm).
Depending on its relationship to the cervix (40), the placenta previa can be classified into three categories: total placenta previa, which covers the entire internal cervical os; partial placenta previa (Fig. 66.2), which covers a portion of the os; and marginal placenta previa, which extends near the os. Evaluating the placental structure has become less important since the introduction of Doppler sonography, which has improved the assessment of placental and fetal perfusion.
In a multiple pregnancy, the placentation of the multiple gestations should be determined. The gestations (95) can have a common placenta («-) (Fig. 66.4) or their own placenta. Furthermore, the expectant parents (and their obstetrician) should be informed of a multiple pregnancy to make preparations for twins (Fig. 66.3) or, as the case may be, triplets (Fig. 66.4). If parents wish to know whether they expect a daughter (Fig. 66.5) or a son (Fig. 66.6), they should be told so, but only if the gender can be unequivocally determined. In early pregnancy, the umbilical cord or a hypertrophic clitoris (\) can easily be mistaken for the penis (\), or the female labia for the scrotum (-») (Figs. 66.5,66.6). The parents should only be told of the gender if they indicate that they wish to know.
The cerebellum (110) is visualized on the transverse section through the posterior cerebral fossa (Fig. 67.1). A dorsal indentation (<-) should be identified. Its absence causes the cerebellum to look like a banana ("banana sign") and indicates cerebellar displacement toward the spinal canal (Fig. 67.2), suggesting a neural tube defect. For the same rea son, the calvaria (105) loses its oval form on transverse sections through the cerebrum and resembles a sliced lemon ("lemon sign") with projections (✓) of the parietal bone bilaterally (Fig. 67.3). Incidentally visualized is the hypere-choic choroid plexus (104).
Cerebrospinal fluid spaces: The choroid plexus can harbor small, unilateral cysts (\ ) (Fig. 67.4), generally without pathologic consequence. Bilateral cysts, however, are associated with trisomy 18 and, less frequently, with renal and cardiac malformations. A hydrocephalus (Fig. 67.5), as seen with aqueduct stenosis or as part of a spina bifida, is accompanied in 70-90% of cases by other intra- and extracerebral malformations. After the 20th gestational week, the ratio of the ventricular to the hemispheric diameter, also called the ventricular index, is used for assessing the ventricles, with a ratio of 0.5 considered indicative of hydrocephalus. The ratio at the level of the frontal horns (AHVR) is slightly exceeded by the ratio at the level of the occipital horn (PHVR) (Fig. 67.6b). Measuring the diameter of the ventricles and hemispheres can be difficult because the lateral ventricular wall often is not clearly demarcated from the cerebral parenchyma (Fig. 67.6a).
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