The midsagittal plane of the fetal head (Figs. 9.3 and 9.4), demonstrating the fetal profile in the first trimester, enables the assessment of the forehead, nose with nasal bone, mouth with maxilla, mandible anteriorly, and NT posteriorly. In the first trimester, the fetal head appears slightly larger in proportion to the body than later on in gestation, and in this midsagittal view the forehead shows some normal-appearing “frontal bossing” (Figs. 9.3 and 9.4). At this stage of early gestation, the metopic suture is still wide and the frontal bones are not seen in the midsagittal view. The perpendicular insonation to the nasal structures in the midsagittal plane of the face enables clear visualization of facial structures. The midsagittal plane landmarks are important in order to correctly identify the nasal bone on midsagittal scanning, with the demonstration of the “equal sign” formed by the nasal bone inferiorly and the nasal skin superiorly (Figs. 9.4, 9.5A, 9.6, and 1.1). The maxilla is recognized in the midsagittal plane as a continuous ossified region in the face (Figs. 9.4, 9.5 and 9.6). In the midsagittal plane, the anterior part of the mandible is seen as an echogenic dot under the anterior maxilla (Fig. 9.6A). In parasagittal views, a larger part of the mandible can be seen (Fig. 9.6C) as well as the maxillary process between the nasal bone and the maxilla (Fig. 9.6B). Several facial measurements in the midsagittal view have been proposed in the literature.², ³, ⁴, ⁵, ⁶, ⁷, ⁸ The significance of abnormal facial measurements and anatomic markers is discussed later in this chapter.

In the posterior aspect of the midsagittal view, the neck with NT is also demonstrated. A detailed discussion of NT measurement (Figs. 9.3 and 9.4) and its significance is presented in Chapters 1 and 6. Conditions associated with a thickened NT and cystic hygroma are discussed at the end of this chapter.

A frontal coronal view of the bony face in the first trimester reveals both orbits and eyes and their relationships to the nasal bridge and maxilla (Fig. 9.7) (see Chapter 5). The position, size, and shape of the eyes and orbits are generally assessed in a subjective manner. A coronal view of the face demonstrates
the anterior maxilla (alveolar ridge) (Fig. 9.7). The retronasal triangle is imaged in an oblique plane, between the coronal and axial planes of the face, in the region of the nose and maxilla (Fig. 9.7B)⁹ (see Chapter 5 and Fig. 5.9). An oblique plane between the maxilla and the mandible normally reveals a mandibular gap (Fig. 9.7B).¹⁰ Good visualization of facial structures is typically performed from a 3D surface rendering of the face, ideally obtained transvaginally.

In the experience of the authors, the systematic visualization of the axial planes of the face is of secondary importance to the midsagittal and coronal planes (Chapter 5). As performed in the second trimester, multiple axial planes obtained in the first trimester from cranial to caudal enable the demonstration of orbits, nasal bridge with nasal bones, the maxilla, and the mandible (Fig. 9.8) (Chapter 5). The fetal lips cannot be well identified on transabdominal scanning in the first trimester, and, when needed, imaging of these structures can be obtained transvaginally with high-resolution transducers.

Similar to the use of three-dimensional (3D) ultrasound in surface mode of the fetal face in the second and third trimesters of pregnancy, 3D ultrasound in the first trimester (Fig. 9.9) provides additional information to the 2D midsagittal (profile), frontal, and axial facial views.¹¹ When fetal abnormalities are suspected in the first trimester, 3D ultrasound of the fetal face in surface mode enables a detailed view of facial features, including the forehead, eyes, nose, mouth, chin, and ears (Figs. 9.9, 9.10 and 9.11). Three-dimensional ultrasound of the



fetal face can often be obtained by transabdominal acquisition, but when an abnormality is suspected the transvaginal approach provides for more details and higher resolution. Figures 9.9, 9.10 and 9.11 show examples of normal and abnormal fetal faces on 3D ultrasound in the first trimester of pregnancy. Three-dimensional ultrasound can also be used in multiplanar display with reconstruction of planes for the specific evaluation of target anatomic regions (Figs. 9.12, 9.13 and 9.14) such as the
bony face or the palate in the evaluation of facial anomalies (see later). For more details on the use of 3D ultrasound in the first trimester, refer to Chapter 3 in this book and a recent book on the clinical use of 3D in prenatal medicine.¹¹

Reference ranges for nasal bone length in the fetus were reported in the second and third trimesters of pregnancy, and nasal bone has been described to be absent or short in fetuses with trisomy 21.¹² This observation was adapted to aneuploidy screening at 11 to 14 weeks of gestation, and Cicero et al.² demonstrated that nasal bones are hypoplastic or not ossified in the first trimester in the majority of fetuses with trisomy 2¹² (Figs. 9.15 and 6.1), and in other aneuploidies and syndromic conditions (Figs. 6.6, 6.8, 6.33, and 6.35).¹³ Assessment of the nasal bone is also used to improve the efficiency of the combined first-trimester screening for Down syndrome.¹³,¹⁴ Table 1.2 in Chapter 1 summarizes the essential criteria for an accurate nasal bone assessment in the first trimester.

The observation that the skin of the forehead, called the “prenasal thickness,” is increased in the second trimester in

fetuses with trisomy 21¹⁵,¹⁶ has led to the use of this marker in the first trimester of pregnancy as well (Fig. 9.16).⁵,⁸,¹⁷ To reduce the false-positive rate of prenasal thickness measurement, the ratio of the prenasal thickness to nasal bone length was proposed⁵ (Fig. 9.16). In normal fetuses, the prenasal thickness is small and the nasal bone is relatively long, resulting in a ratio of approximately 0.6.⁵ In trisomy 21 fetuses in the first trimester, the prenasal thickness increases, whereas the nasal bone length decreases, resulting in a ratio >0.8.⁵

Fetuses with trisomy 21 have a flat profile due to midfacial hypoplasia, leading to the known feature of a protruding tongue. Measuring the maxillary length between 11 and 14 weeks of gestation is proposed as a method to quantify midfacial hypoplasia.³ The measurement is performed in a slightly parasagittal view of the facial profile and includes the mandibular joint.³ Maxillary length is short in fetuses with trisomy 21.³ Midfacial hypoplasia can also be assessed by the use of the frontomaxillary facial angle, which indirectly includes the maxilla.¹⁸ The frontomaxillary facial angle is discussed in the next section.

The frontomaxillary facial (FMF) angle is the angle between the maxilla and forehead and in normal fetuses is quantified at 85° (±10°) (Fig. 9.17A).¹⁸ A wide FMF angle is reported in fetuses with trisomy 21⁴,¹⁹ (Fig. 9.17B), whereas a narrow FMF angle is noted in fetuses with open spina bifida (Fig. 9.18).²⁰ Abnormal FMF angles are also reported in fetuses with trisomy 18 with midfacial hypoplasia and micrognathia²¹ and in fetuses with trisomy 13 in association with holoprosencephaly.²² Caution should be used, however, in the evaluation of the FMF angle because slightly oblique views will introduce false-positive and negative results. Measurement of the FMF angle using 3D multiplanar rendering has been shown to improve its accuracy.¹⁸,²³ The wide FMF angle in fetuses with aneuploidies is likely due to the short maxilla, whereas in spina bifida the narrow angle is probably due to the small flat head owing to the posterior shift of the brain and decreased fluid in the ventricles. Another facial angle is the maxilla-nasion-mandible (MNM) angle and uses the nasion as reference point of the intersection of the frontal and nasal bones.⁸,²⁴ The MNM angle is defined as the angle between the maxilla-nasion line and the mandible-nasion line in the midsagittal plane of the face and can be used to identify fetuses at high risk for aneuploidies, micrognathia, and clefts.⁸

Prefrontal space distance (PSD) is obtained by drawing a line from the anterior aspect of both the mandible and maxilla and extended toward the fetal forehead (Fig. 9.19).⁶,⁷ The

PSD is calculated by the distance of the prenasal skin to this extended line (Fig. 9.19). The distance can have positive or negative values.⁶ PSD is abnormal in fetuses with aneuploidies, such as trisomies 21, 18, and 13,⁶ as well as in fetuses with micrognathia and clefts⁷ (Fig. 9.19).

To the best of our knowledge, no charts currently exist on the size of the orbit and the interorbital distances in the first trimester of pregnancy, and such measurements are not obtained routinely. Recently, a paper reported on the interlens distance, starting at 12 weeks of gestation.²⁵

Trisomy 21 fetuses typically show an abnormal facial flat profile with an absent or hypoplastic nasal bone (Fig. 9.15), a short maxilla, an increased FMF angle (Fig. 9.17), and a thickened prenasal thickness (Fig. 9.16). Similar facial appearance can also be found in trisomy 18 fetuses, in addition to retrognathia and facial clefts. Trisomy 13 fetuses show severe facial anomalies due to their association with holoprosencephaly (Fig. 9.20) and/or with facial clefts. Ultrasound markers of aneuploidies, including facial abnormalities in the first trimester, are discussed in detail in Chapter 6.

Lobar and semilobar holoprosencephaly is often associated with facial abnormalities such as cyclopia, hypotelorism, proboscis, cebocephaly, agnathia-holoprosencephaly, nasal hypoplasia, and facial clefts.²⁶ In most cases, the profile is severely abnormal, in addition to the abnormal head shape and brain. Figures 9.10 and 9.20 show abnormal profiles in fetuses with alobar holoprosencephaly. Holoprosencephaly is discussed in detail in Chapter 8.

In acrania/anencephaly/exencephaly, the profile and the frontal view of the face have characteristic abnormalities with the presence of large eyes and small face. Facial profile views in Figure 9.21 show different aspects of the forehead region in acrania. Abnormalities in facial profiles in anencephaly/exencephaly are discussed in detail in Chapter 8.