The anatomical survey, as well as the understanding of the developmental anatomy and detection rates of fetal anomalies, has evolved over the last 30 years as the result of more advanced and sophisticated ultrasound technologies. Over the last 25 years, 18 weeks has been considered by many to be the ideal or “gold-standard” gestational age to perform the first, and at times the only, anatomical survey. There are numerous advantages to performing the scan at this gestational age: the fetus is large enough to be imaged easily using transabdominal (TA) sonography, and most anomalies are present and can be detected. The disadvantage of using this gestational age for the anatomical survey is that maternal body characteristics such as abdominal obesity, uterine fibroids, and fetal position can at times preclude adequate imaging of the fetal structures, which can result in an incomplete scan. This, in turn, necessitates a follow-up scan. This need for follow-up scans has resulted in a gradual shift of the 18-week anatomy scan to be performed at 20 to 22 weeks. Therefore, the “18-week anatomy scan” has become a brand name for the sometimes only detailed anatomical survey that in many practices is carried out typically at around 20 weeks or anytime between 18 and 22 weeks of gestation.

Many of the fetal anomalies seen at the 18-week anatomy scan are present starting at the first and/or early second trimesters and, if carefully looked for, can be detected. In the 1990s, the early anatomy scan was performed mostly using transvaginal sonography (TVS) between 14 and 17 weeks of gestation. Although during this scan the normal early fetal anatomy can be seen and anomalies can be detected, it never reached popularity, and few practices adopted it as a routine¹. The “11- to 13-week scan” also known as the “nuchal scan” is part of the first trimester screening for Down syndrome and continues to have increasing acceptance among patients and practitioners even in the era of noninvasive prenatal screening (NIPS) using cell-free fetal DNA from maternal blood. In 1997, D’Ottavio et al² in a study a total of 3514 fetuses reported a detection rate of a single scan at 13-15 weeks to be 66.7%, and in combination with a 20-22 weeks scan its 90.5%. In another study, this time a prospective observational study, which aimed to determine the efficacy of the first-trimester anomaly scan, Becker and Wegner³ scanned 3094 consecutive fetuses between 11 and 13 weeks and 6 days with an 83.7% detection rate of major anomalies. Ebrashy et al⁴ in prospective study of 2876 patients found that all anomalies that were detected at the first- and/or midtrimester scans were confirmed and documented after delivery. At the 13- to 14-week scan they diagnosed 21 (68%) of the 31 cases diagnosed with anomalies prenatally. Recently, Bromley et al⁵ reported a 41.4% detection rate of malformations at 11 to 13 weeks and 6 days without even having a dedicated scanning protocol; furthermore, at this gestational age they were able to diagnose 71% of the lethal anomalies. Therefore, based upon these publications it is important to conclude that by 12 weeks approximately 40% to 50% of frequently discernible sonographic anomalies are already present and up to 80% of lethal and major fetal anomalies can be detected by skilled sonologists.

In this chapter, we will use the “18-week anatomy scan” as a metaphor for an anatomy scan performed anytime between 18 and 22 weeks of gestation. The term “the NT scan” will encompass the scan performed at 11 to 13 weeks and 6 days at the time of the nuchal translucency unless otherwise specified. The term “early anatomy scan” applies to those scans performed between 14 and 17 weeks and 6 days. It is important to stress from the outset that an anatomy scan performed either at the NT scan or early anatomy scan should always be followed by the 18-week anatomy scan to achieve the highest possible combined detection rates of fetal anomalies.

Before performing an anatomical survey, regardless of the gestational age, several questions need to be answered: What structures should be included in the scan? Which anomalies can be reliably diagnosed at the gestational age when the scan is being performed?

The first trimester anatomical scan can ideally be performed at 11 to 13 weeks and 6 days (crown rump length [CRL] 45-84 mm) in conjunction with the nuchal translucency.^6,7 This scan can be performed using both TA and/or transvaginal (TV) ultrasound alone or in combination with technologies such as three-dimensional (3D) and four-dimensional (4D) ultrasound. In the past, early fetal anatomical scanning relied on TV probes due to their high frequencies ranging from 6 to 12 MHz. High frequency transducers produce high resolution pictures which allow imaging of the small developing fetus, at the expense of a relatively shallow penetration; therefore, these probes need to be placed in close proximity to the developing fetus. The ideal way to achieve this is to use the TV scanning approach. The dependence of this early scan on the vaginal approach may be one of the main reasons why first and early second trimester fetal anatomical scanning has not gained wide acceptance. Recent significant improvement in TA imaging probe technology has resulted in better performance of higher frequency TA probes, ranging from 2 to 5 MHz, that allow excellent diagnostic images of the first and early second trimester fetus almost equaling the resolution of TV probes. Operators not keen to perform TV imaging can now carry out a detailed scan at this early gestational age using the TA route in most cases. Additional reasons for the hesitancy in performing an early anatomy scan have been the lack of understanding of early fetal developmental anatomy and embryology, the rising obesity rate in the United States⁸ limiting TA route imaging⁹; and in addition some insurance companies do not routinely reimburse for an early anatomy scan in addition to the “gold-standard” second trimester anatomy scan.

Among the first studies looking in a systematic fashion at the anatomy of the first trimester fetus was that authored by Timor-Tritsch et al¹⁰ in which 97 low-risk patients were scanned between 9 and 14 weeks using TVS. The aim of the study was to assess at what gestational age fetal structures such as body contours, long bones, fingers, face, palate, feet, toes, and the four-chamber view could consistently be imaged. The results of the study showed that by 13 to 14 weeks all of the structures targeted in the study could consistently be imaged (Table 5-1). In 1998 Whitlow et al¹¹ published a study on the optimal gestational age to measure the nuchal translucency and simultaneously examine the fetal anatomy. The study results showed that week 13 of the gestation was the ideal time to perform a nuchal translucency measurement and image the basic fetal anatomy. The study also showed that with increasing gestational age during the nuchal lucency window the percentage of the cases in which the anatomy could be seen increased from 75% at 11 weeks to 98% at 13 weeks. Interestingly, they also noted that as gestational age increased the need for scanning using TVS decreased from 42% at 11 weeks to 15% at 13 weeks.

Subsequently, in 2004 Souka et al¹² published on the feasibility of examining cardiac and noncardiac fetal anatomy in 1144 low-risk women between 11 and 14 weeks. The scan was performed using both TA as well as TV imaging of the following anatomical structures: skull, brain, face, spine, and four-chamber and three-vessel views of the heart, stomach, abdominal wall, kidneys, bladder, and extremities. This study concluded that a complete anatomy scan was possible in 48% of the fetuses between 11 and 14 weeks. However, if cardiac anatomy was excluded, successful imaging of noncardiac anatomy was possible in 86% of the fetuses. Using TV imaging increased the successful examination rates of fetal anatomy from 72% to 86% of the fetuses. In this study, TV imaging was for the most part helpful in examining the face, kidneys, and bladder. Similar to other studies, this study demonstrated the role of the crump-rump-length (CRL) rather than gestational age alone, as the CRL increased so did the visualization rates of fetal structures.

In the United States, in contrast to most European and South American countries where the obstetrical scan is performed by dedicated physicians (sonologists), mostly obstetricians, the role of scanning has been delegated to dedicated and highly skilled sonographers. In 2004, Timor-Tritsch et al¹³ in a prospective cross-sectional study of 223 women between 11 to 13 weeks and 6 days evaluated the ability of sonographers to perform fetal structural evaluation at 11 to 14 weeks, using both TA and TV imaging. The sonographers were asked to look for fetal structures of the head, neck, spine, heart, abdomen, chest, and extremities; in total 37 structures were evaluated. The cases were divided into groups by gestational age: 11 to 12 weeks and 13 to 14 weeks. In this study, similarly to prior studies, as the gestational age increased so did the number of structures imaged. The cardiac views had the lowest percentage rate of visualization which is not surprising since the fetal heart is among the most difficult fetal structures to image. The authors concluded that anatomic survey of the fetus between 11 and 14 weeks can be performed by sonographers with good detection rates of most fetal structures (Table 5-2).

A recent systematic review by Rossi AC et al¹⁴ looked at the usefulness of ultrasound between 11 and 14 weeks in the diagnosis of fetal structural malformations. Nineteen articles with a total of 78,002 fetuses undergoing fetal anatomical survey at 11 to 14 weeks were included in the meta-analysis. A total of 996 fetuses had ultrasound detected fetal structural malformations resulting in a prevalence of 12 malformations per 1000 fetuses. The detection rate of anomalies in this systematic review was 51%, with detection rates increasing to 62% when both TA and TV imaging were included. Additionally, the detection rate increased to 65% among those patients who were at high risk for malformations.

A prospective longitudinal study by Yagel¹⁵ et al published in 2015 aimed to compare visualization rates for early targeted organ scanning using TA sonography at 11 weeks and 3 days to 13 weeks and 2 days versus 14 weeks and 3 days to 16 weeks and 2 days. The results demonstrated that overall visualization rates for all structures between 14 weeks and 3 days to 16 weeks and 2 days were more than 94%; however, visualization rates of the structures at 11 weeks and 3 days to 13 weeks and 2 days were not consistent, showing significant variability (Table 5-3). They concluded that an early anatomical scan between 14 weeks and 3 days to 16 weeks and 2 days is more productive as far as higher visualization rates of fetal structures than a scan performed at 11 weeks and 3 days to 13 weeks and 2 days.

Bronshtein et al¹⁶ published on their experience of performing a systematic TV scan at 14 to 17 weeks for fetal organ screening. A total of over 72,000 scans were performed between 14 and 17 weeks (early) and between 18 and 24 weeks (late) 96% of the malformations were detectable in the early screening with an incidence of 1 in 50 gestations. In this study, only 4% of the fetal anomalies were diagnosed later in pregnancy. They concluded that it is feasible to perform an anatomical survey by an expert sonographer who has been trained in the detection of fetal malformations, at 14 to 17 weeks’ gestation.

A study by Le Lous M et al¹⁷ in a cohort of 166 fetuses with NT ≥3.5 mm with normal karyotype and known outcome assessed the utility of an additional scan at 16 weeks. In their cohort 51 (30.7%) of the fetuses had a structural defect identified during the pregnancy. In the first trimester the NT scan alone was useful in identifying 16 of the major defects (31.3%); the 16-week scan was useful in identifying an additional 21 major anomalies (41.2%), whereas the 22nd-week scan discovered an additional 14 major malformations (27.4%). Seventy-two percent of the major structural defects were detected before the 22-week scan.

Given this background, the following question arises: Which fetal anatomical structures should be sought in the fetal anatomical survey during the NT scan or in the early anatomical survey? Before setting out to scan a fetus in the late first or early second trimester, it is important to establish the correct gestational age of the fetus; ideally, the gestational age has been determined at a prior scan. Correct gestational age is important because not all structures that are part of the 18 to 22 weeks’ scan have completed their normal development at the usual time for an NT scan or the early anatomy scan. In addition, not all fetal structures mature at the same time or at the same gestational age. An important example is that of the fetal brain, which undergoes continual structural changes during embryonic/fetal life. Depending on the gestational age, certain fetal neurologic structures may appear abnormal or pathologic, while in reality they have not yet complete their normally scheduled development. For example, early in the first trimester, a difference of 5 to 7 days in the gestational may lead to misdiagnosing a normally developing ventricular structure such as the rhombencephalon as ventriculomegaly or nonvisualization of the falx as holoprosencephaly. While at the early anatomy scan at 14 to 16 weeks, nonvisualization of the cavum septi pellucidi and corpus callosum may be misdiagnosed as agenesis of these structures, when in fact this is a normal gestational developmental phenomenon.

Therefore, when performing a fetal anatomical survey the American Institute of Ultrasound in Medicine (AIUM)¹⁸ published a list of structures that constitute a fetal anatomical scan at 18 to 20 weeks; however, this guideline cannot be applied to the fetus at 11 to 13 weeks and 6 days or at 14 to 17 weeks and 6 days. At present, only the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG)¹⁹ published guidelines of what structures have to be included during the NT scan (Table 5-4), though there are no guidelines regarding the fetal structures that constitute the early anatomy scan at 14 to 17 weeks and 6 days.

The ALARA principle (as low as reasonably achievable) is the principle assuring biosafety in pregnancy, in which diagnostic-quality images are obtained while exposing the fetus to the lowest acoustic energy output for the shortest amount of time. The output display standard (ODS) consists of the thermal index (TI) and the mechanical index (MI). These indexes are displayed on the ultrasound monitor screen. The TI denotes the possibility for a rise in temperature at the ultrasound’s focal point, and the MI expresses the likelihood for the ultrasound to cause cavitation in tissues.²⁰ During the first trimester, the thermal index for soft tissue (TIs) should be used in pregnancies of less than 10 weeks’ gestation. After 10 weeks, or when bone ossification is present, the thermal index for bone (TIb) should be used.¹⁸ Pulsed Doppler including spectral, power, and color flow should not be part of the routine scan in the first trimester. However, it may be used for clinical indications. The TI should be less than or equal to 1, and exposure time ought to be kept as short as possible.²¹

All scans begin assessing viability by obtaining a fetal heart rate and then determine the number of fetuses and perform biometry to assure correct dating and growth of the fetus. Biometry at this gestational age includes crown-rump-length (CRL), and additional measurements performed at this gestational age include nuchal translucency. Maternal structures such as the uterus and adnexa are also part of the scan. The cervix, when indicated, should be assessed as well.

The Fetal Head

The head is usually imaged in an axial section. The cranium should be seen as a bright echogenic structure encasing the brain. The interhemispheric fissure and falx cerebri divide the brain into 2 hemispheres of equal size. The normal lateral ventricles appear relatively large and are surrounded by a thin rim of brain the tissue. The choroid plexuses are filling the lateral ventricles; their shape has been compared to a butterfly with the wings open²² (Figure 5-1A). The posterior fossa can be imaged at this gestational age; however, many of the structures are not fully developed at this time (Figure 5-1B). In the sagittal section, the nuchal translucency can be seen and should be measured. In this section, the integrity of the cranium, the fetal profile, nasal bone, palate, and the mandible can be assessed (Figure 5-1C). The palate can be further evaluated using an anterior coronal view of the fetal face on which the retronasal triangle is seen (Figure 5-1D). The apex of the retronasal triangle is made up of the 2 nasal bones; the sides are the frontal process of the maxilla, and the base of the triangle is the primary palate, which is the most anterior part of the palate.^23,24 Chaoui et al²⁵ described a new sign, the maxillary gap, which is a visible gap or defect seen in the palate in the sagittal plane of the face, the same plane used to measure the nuchal translucency (Figure 5-1C). The conclusion of their retrospective study was that using this sign could enhance the detection rate of cleft lip± palate, especially in isolated cases. The lips and nose can be imaged in an anterior coronal section, similar to that used during the second trimester anatomy scan. The lenses can be seen using an anterior coronal or axial section of the face at the level of the orbits. The normal lens appears as a ring with an anechoic center and an outer echogenic rim (Figure 5-1E). The hyaloid artery can at times be seen as a linear structure between the posterior aspect of the lens and the optic disc. Adding 3D and 4D sonography helps in evaluating the surface anatomy of the face at this gestational age (Figure 5-1F).