Continuing Education Activity

The mid-trimester ultrasound (also known as the second-trimester anatomy scan or the anomaly scan) is a routine examination in many countries that's aimed primarily to assess fetal anatomy and detect any fetal anomalies. A two-dimensional grayscale ultrasound with a curvilinear transabdominal probe is routinely used to evaluate the fetal number, viability, gestational age, anatomical survey, placental location, amniotic fluid, and maternal pelvic organs. A transvaginal probe, color Doppler and three-dimensional ultrasound are not routinely used but may be implemented in certain clinical scenarios, especially when placenta previa or fetal anomaly are suspected, or to provide accurate cervical length measurement. This activity outlines the indication, safety, the technique of second-trimester ultrasound and highlights the role of the interprofessional team in conducting and interpreting the study to improve the antenatal care provided.

Objectives:

• Describe the scanning protocol used in the anatomy scan systemically.
• Summarize the indications, timing, and safety of the second-trimester ultrasound scan.
• Identify the basic requirement needed to conduct the scan.
• Explain how interprofessional team strategies can optimize patient outcomes when using ultrasound for pregnancy evaluation.

Introduction

Antenatal ultrasonography is widely used in pregnancy to assess fetal growth and anatomy. Although ultrasound screening is now an integral part of routine antenatal care, recommendations for the delivery of obstetric ultrasound vary from country to country.[1] The history of sonography in Obstetrics dates from the classic 1958 Lancet paper of Ian Donald and his team from Glasgow. Clinical researchers have seized technological developments such as real-time imaging, color, and power Doppler, transvaginal sonography, and 3/4D imaging to enhance the investigation and management of patients in areas as diverse as assessment of fetal growth and wellbeing, screening for fetal anomalies, and ultrasound-guided procedures as an essential component of fetal therapy.[2]

The diagnosis of a fetal anomaly significantly reduces perinatal mortality and morbidity and maternal morbidity. Prenatal diagnosis enables a psychologically less traumatic and early medical termination of pregnancy. It thereby decreases the probable complications of continuation of pregnancy and labor, prevent an unnecessary cesarean section for a fetus with lethal anomalies diagnosed too late for medical termination of pregnancy, and allow planning delivery at the optimal time in a well-equipped tertiary center with necessary neonatal care facilities, and in utero therapy in selected cases. Performed systematically, high-resolution ultrasound can now accurately diagnose more than 200 abnormalities. A normal scan is often obtained, but there is tremendous relief of psychological distress, anxiety, and somatic symptoms after the report.[3] The literature includes descriptions of anatomical surveys performed before 18 weeks, but other studies have repeatedly shown that more anomalies are diagnosed if the scan is done after 18 weeks.[4][5]

The detection rate of the screening second-trimester ultrasound scan was good in high-risk patients examined by the well-trained sonographer. On the other hand, in low-risk pregnant women, the scan could also be sensitive in anomaly detection if a systemic searching pattern was followed.[6]

Indications

An ultrasound scan performed between 18 and 22 weeks gestation provides the pregnant woman and her care provider with information about multiple aspects of her pregnancy. The obstetrical ultrasound will inform them of and/or confirm the number of fetuses present, the gestational age, and the location of the placenta. It will present an opportunity to diagnose congenital anomalies and/or to detect soft markers of aneuploidy and to identify maternal pelvic pathology.[7]

Contraindications

Based on evidence currently available, routine clinical scanning of every woman during pregnancy using real-time B-mode imaging is not contraindicated. American Institute of Ultrasound in Medicine (AIUM) and National Electrical Manufacturers Association (NEMA) regulated that the ultrasound device has to display the emitted energy through the thermal index (TI) and the mechanical index (MI). Both indexes are not perfect indicators of the risks of thermal and non-thermal bioeffects, but currently, they should be accepted as the most practical and understandable methods of estimating the potential for such risks.

Acoustic outputs in B-mode and M-mode are generally not high enough to produce deleterious effects. The significant temperature increase may be generated by spectral Doppler mode, particularly in the vicinity of a bone. This should not prevent the use of this mode when clinically indicated, provided the user has adequate knowledge of the instrument’s acoustic output or has access to the relevant TI. Exposure time and acoustic output should be kept to the lowest levels consistent with obtaining diagnostic information and limited to medically indicated procedures, rather than purely entertainment purposes. This unequivocal demand of carefulness is best expressed by the ALARA principle (i.e., as low as reasonably achievable).[8][9]

Equipment

The examination is mostly performed with gray-scale 2D ultrasound. Harmonic imaging may enhance the visualization of subtle anatomic details, particularly in patients who scan poorly. High-frequency ultrasound transducers increase the spatial resolution but decrease the penetration of the sound beam. The optimal transducer and operating frequency choice are influenced by several factors, including maternal habitus, fetal position, and the approach used. Most basic examinations are satisfactorily performed with 3–5-MHz transabdominal transducers.[10]

Personnel

The scan should possibly be performed according to international standards and by accredited sonographers who have completed appropriate training programs by scientific societies.[10]

A sonologist, a physician with training and experience in this field, supervises and interprets the exams. It is the physician's responsibility to write a report based on the sonographer's data and, if necessary, personally scanning the patient to validate or change the differential diagnosis.

Preparation

• Reason for examination: the sonographer should validate the indication for the ultrasound examination. The last menstrual period (or the estimated delivery date) should be documented. All this information is critical in targeting specific structures. Choosing whether to use a transvaginal and/or transabdominal technique and whether additional studies may be helpful (e.g., Doppler velocimetry).
• Patient positioning: a semirecumbent position is the most commonly used in obstetrics and gynecology. A padded table and pillow are used to ensure maximum comfort. It is preferable to elevate the head of the bed as some pregnant women are unable to lie flat, especially in late pregnancy. For the transvaginal exam, a lithotomy position is used.
• Special consideration for the obese patients: in obese individuals, imaging could be improved by placing the transducer on the side rather than on the midline and while the patient is lying on her side. The transvaginal probe is also helpful in those patients.[11]
• Bladder filling: This is useful when the lower uterine segment is of interest. It is often of little benefit in obstetric ultrasound examination as it can give a false diagnosis of placenta previa or elongated cervix. Transvaginal sonography is usually performed with an empty bladder.

Technique

A. Fetal biometry and wellbeing: The following sonographic parameters can help to estimate gestational age and fetal size:[12][13][14]

• Biparietal diameter (BPD) /head circumference (HC): measured at a cross-sectional view of the fetal head at the level of the thalami and cavum septi pellucidi without visualization of the cerebellum. BPD is commonly measured from the leading edge (outer edge of the proximal skull) to the leading edge (inner edge of the distal skull), while HC is measured around the outside of the skull bone echoes. When the head shape is flattened (dolichocephaly) or round (brachycephaly), HC is more reliable than BPD.
• Abdominal circumference (AC) or diameter: can be measured, using ellipse function, at the outer surface of the skin on a transverse section of the fetal abdomen at the junction of the umbilical vein, portal sinus, and stomach.
• Femur length (FL): measured along the longest axis of the ossified diaphysis but excluding distal femoral epiphysis.

Measurements should be performed in a standardized manner based on strict quality criteria (figure1).[15] Reference standards appropriate for the local population should be used to interpret the measurement results. BPD and/or HC are preferable to other growth parameters in the estimation of gestational age.[16][17]

B. Anatomical survey: Minimal requirements (figure2) and optional views (figure3) for the routine mid-trimester anomaly scan are summarized in Table 1[14]

Table 1: Minimal requirements and optional views for the routine mid-trimester anomaly scan

1. Head: including skull and brain:

• Skull: size, shape, integrity, and bone density require routine evaluation. All these characteristics can be visualized at the time of the head measurements. Size measurements are performed as mentioned in the biometry section. The skull normally has an oval shape without focal protrusions. Alterations of shape (e.g., lemon, strawberry, cloverleaf) should be documented and investigated. Normal skull density is manifested as a continuous echogenic structure interrupted only by cranial sutures in specific anatomical locations. The absence of this whiteness or extreme visibility of the fetal brain should raise suspicion of poor mineralization (e.g., osteogenesis imperfecta, hypophosphatasia). Poor mineralization is also suggested when the skull becomes easily depressed as a result of manual pressure from transducer placement against the maternal abdominal wall.[18]
• Brain: The axial planes that allow the assessment the anatomic integrity include (Figure 4):
• Transventricular plane: demonstrates the anterior and posterior portion of the lateral ventricles. The frontal horns (anterior portion) appear as two comma-shaped fluid-filled structures separated medially by the cavum septi pellucidi (CSP), a fluid-filled cavity. The CSP becomes visible around 16 weeks and undergoes obliteration near term or in the early neonatal period. It should always be visualized between 18 and 37 weeks. The value of visualizing the CSP for identifying cerebral anomalies has been debated. However, this structure is altered with many cerebral lesions such as holoprosencephaly, agenesis of the corpus callosum, severe hydrocephaly, and septo-optic dysplasia. The posterior portion of the lateral ventricles is a complex formed by the atrium that continues posteriorly into the occipital horn. In a normal fetus, each atrium is usually measured up to 10 mm in transverse diameter; otherwise, it is usually followed to rule out early ventriculomegaly.[10]
• Trans-cerebellar plane: This plane is inferior to the BPD plane, with the probe tilted backward into the posterior fossa. The plane is correct when one can visualize the thalami and cavum septum pellucidum in the same plane as the cerebellum. The cerebellum is a dumbbell-shaped structure with symmetrical lobes. The central vermis is slightly more echogenic than the lateral lobes. The trans-cerebellar diameter (the widest measurement across the cerebellum perpendicular to the falx) in millimeters correlates with the gestational age up to 20 weeks and is larger than gestational age after this time. A cerebellum measuring 2 mm less than gestational age is a concerning finding. The cisterna magna can be measured from the posterior margin of the cerebellar vermis to the inside of the occipital bone in the midline. A measurement of 2–10 mm is normal in the second and third trimesters. The nuchal fold is a measurement taken from the outer skin line to the outer bone in the midline. Less than 6 mm is considered normal up to 22 weeks.[19]
• Trans-thalamic plane: The anatomic landmarks include, from anterior to posterior, the frontal horns of the lateral ventricles, the cavum septi pellucidi, the thalami, and the hippocampal gyruses.[20]

Example of fetal head abnormalities:[21]

• Thickened nuchal skin fold: Assessment of the nuchal fold is typically made using a trans-cerebellar plane. A nuchal fold thickness of greater than 6 mm is considered abnormal and is seen in 80% of newborns with Down syndrome.
• Cystic hygroma: a localized, single, or loculated, fluid-filled cavity usually occurs in the neck. It is seen in fetuses with nonimmune fetal hydrops, Turner syndrome, and trisomy syndromes 13, 18, and 21. Detection of a cystic hygroma should prompt performance of amniocentesis since the prevalence of cytogenetic abnormalities has been reported as high as 73%.
• Meningoceles are seen as fluid-filled cystic structures (figure5) and encephaloceles (figure6) as brain-filled cystic structures that extend through a bony calvarial defect, usually in the occipital or frontal region. The entity most commonly mistaken for a small meningocele is a cystic hygroma. Careful scanning, however, will reveal no calvarial defect with a cystic hygroma. The absence of brain tissue within the sac is the single most favorable prognostic feature. Associated anomalies include Arnold-Chiari malformation, Dandy-Walker syndrome, and Meckel-Gruber syndrome (encephalocele, microcephaly, polydactyly, and cystic dysplastic kidneys).

2. Face/neck: early detection of facial abnormalities may lead to the diagnosis of multiple syndromes, which allows better parents' counseling and optimal care. A coronal view is usually used to assess the nose and lips. The biorbital and interorbital distances are measured by obtaining a transverse view at the level of the orbits, while a sagittal view is used to assess the facial profile and the cervical spine.

Example of face/neck abnormalities:

• Nasal bone absence/ hypoplasia: in the second trimester, hypoplastic nasal bone must be considered as important as an absence. Trisomy 21 is associated with nasal bone absence/ hypoplasia in around 40% to 80%. [22]
• Hypertelorism and hypotelorism: the distance between the two inner canthi, known as interocular distance (IOD), approximates the ocular diameter (specific charts are used). While the distance between the two outer canthi known as binocular distance (BOD). Hypertelorism (increase in IOD and BOD) may be associated with frontal encephaloceles, craniosynostoses, exposure to phenytoin, and cleft lip and palate. Hypotelorism (reduced IOD and BOD ) is usually associated with holoprosencephaly and other brain malformation.[21]
• Cleft lip and palate are the most common face malformations. Other congenital anomalies are seen in about half of the cases[21] 
• Neck masses include teratoma, lymphangioma, enlarged thyroid, branchial cleft cyst, and rarely a sarcoma.[21]

3. Thorax: The shape of the thorax, ribs, and homogeneous echogenicity of both lungs can be examined in axial views. The integrity of the diaphragm can be examined through the sagittal or coronal view. For the basic cardiac screening, a proper four-chamber view of the fetal heart should be obtained with the assessment of the following: the heart rate (from 120 to 160 beat per minute), location (in the left chest), axis (about 45° toward the left side of the fetus), and size (not larger than one-third the area of the chest), two atria, two ventricles, two atrioventricular valves. Abnormalities like mediastinal shift, lung masses, pleural effusions, diaphragmatic hernia, tachycardia, dextrocardia, cardiomegaly, pericardial effusion, ventricular septal defect, and abnormal chamber size can be detected. Cardiomegaly can be due to congenital heart disease, severe fetal anemia, or a small chest.[14]

4. Abdomen: organ situs should be determined. The fetal stomach should be identified in its normal position on the left side. The fetal umbilical cord insertion site should be examined for evidence of a ventral wall defect. Normal kidneys, along with the adrenal glands, are seen on either side of the spine just below the level of the fetal stomach.[18] The kidneys appear echogenic in the early weeks and gradually become hypoechoic compared with the adjacent bowel and liver. Fetal ureters are not usually visible antenatally unless they are dilated. Persistent absence of the bladder should be considered abnormal from 15 weeks.[23]

Example of fetal abdomen abnormalities:

• Echogenic bowel: defined as bowel with an echogenicity equal to or greater than that of surrounding bone; the differential diagnosis for this finding is broad and includes normal variant, congenital viral infection, cystic fibrosis, aneuploidy, and intra-amniotic bleeding. Previous reports have also suggested an increased incidence of intrauterine growth restriction (IUGR) and intrauterine fetal demise in fetuses with echogenic bowel.[24]
• Fetal bowel dilatation: characterized by fluid-filled intestinal loops which measure at least 15 mm in length or 7 mm in diameter. Dilated fetal bowel is a sign of intestinal mechanical or functional obstruction. Its prevalence will depend on the underlying condition: bowel atresia or stenosis, malrotation with volvulus, meconium ileus, total colonic aganglionosis, and meconium plug syndrome.[25]
• Dysplastic kidneys: result from abnormal development of the glomeruli and nephrons along with a disproportionately increased stroma. They appear large and bright, usually with cystic spaces. Typically, the cysts are multiple, thin-walled with no connections, randomly placed in the renal parenchyma, and form an irregular outlined kidney. Rarely, dysplastic kidneys appear uniformly echogenic on ultrasound without cysts and can be difficult to differentiate from normal kidneys. The renal pelvis and ureters are not seen. Dysplastic kidneys are often associated with other syndromes such as VACTERL association, Meckel–Gruber syndrome, Fraser syndrome, and CHARGE syndrome. Most babies with the isolated unilateral multicystic renal disease tend to have a good outcome. The size and number of cysts in unilateral multicystic disease do not influence the outcome. Bilateral involvement is generally associated with severe oligohydramnios and has a poor prognosis owing to the resultant pulmonary hypoplasia.[23]
• Infantile polycystic kidney disease (Potter type I) has an autosomal recessive inheritance. It carries a 25% risk in subsequent pregnancies. The age of onset of this condition is varied and is subdivided into perinatal, neonatal, infantile, and juvenile. The perinatal onset type (figure8) is the most common and characterized by bilaterally enlarged and homogenously hyperechogenic kidneys, with or without oligohydramnios, renal failure occurring in utero, and 40% to 50% affected with hepatic fibrosis. Kidneys are affected at an earlier onset, and hepatic fibrosis is common in late-onset polycystic kidney disease. Perinatal mortality is usually caused by pulmonary hypoplasia after severe oligo- or anhydramnios.[23]
• When evaluating urinary bladder distention and pyelectasis in the male fetus, the presence of severe or progressive oligohydramnios, progressive bladder wall thickening, and a persistently dilated posterior urethra are most consistent with posterior urethral valves. Although oligohydramnios, dilated posterior urethra, and bladder wall thickening can be seen in prune belly syndrome, these findings are likely to be transient. Vesicoureteral reflux, ureterovesical junction obstruction, and non-refluxing, nonobstructive megacystis–megaureter should be considered when pyelectasis and megacystis are present without additional bladder, urethral, or renal abnormalities.[26]
• The two most common congenital abdominal wall defects are omphalocele and gastroschisis. Omphalocele appears as an outpouching of the abdominal wall recovered by an inner membrane of the parietal peritoneum and an outer layer of amnion and Wharton jelly. It usually occurs at the base of the umbilical cord, the cord insertion being located at the apex of the herniated sac, which contains variable amounts of different visceral organs, usually, bowel or liver or both; concurrent malformations are described in up to 74% of fetuses. It is considered the consequence of a failure of bowel loops to return to the body cavity after their normal physiological herniation into the umbilical cord during the sixth to tenth week of development. Gastroschisis (figure9) is characterized by a full-thickness abdominal wall closure defect, which permits evisceration of the fetal abdominal contents, specifically, herniation of the bowel. The orifice of the defect is usually small and nearly always, is right-sided to umbilicus insertion. There is no surrounding membrane or sac, and these free-floating loops of the intestine may become edematous in consequence of direct exposure to the amniotic fluid. Children of young mothers are more susceptible. It is not usually associated with other anomalies, but sometimes it may occur combined with congenital heart disease, ectopia, cordis, neural tube, and diaphragmatic defects. Survival rates are good (85% to 97%).[27]

5. Spine: The choice of the scanning planes used to evaluate the integrity of the spine depends upon the fetal position. However, a longitudinal view should always be obtained because it may reveal other spinal malformations, including vertebral abnormalities and sacral agenesis. In sagittal planes, the ossification centers of the vertebral body and posterior arches form two parallel lines that converge in the sacrum. In the second and third trimesters of gestation, the conus medullaris is usually found at the level of L2-L3. The integrity of the neural canal is inferred by the regular disposition of the ossification centers of the spine and the presence of soft tissue covering the spine (figure10).[10]

6. Extremities: assessment of all limbs, including the hands and feet, should be done to evaluate the bone shape, size, and integrity. Most prenatal-onset skeletal dysplasias present with a relative disproportion of the skeletal measurements compared with those of the cranium. One of the most important determinations that must be made by ultrasound is that of neonatal or infantile lethality. Lethality occurs in most skeletal dysplasias as a result of a small chest circumference and resultant pulmonary hypoplasia. Using ultrasound criteria for lethality, chest-to-abdominal circumference ratio of <0.6 and femur length-to-abdominal circumference ratios of <0.16 are strongly suggestive of lethality.[28]

Examples of fetal skeletal dysplasia:

• Thanatophoric Dysplasia: is the most common lethal dysplasia. Inheritance is generally autosomal dominant. It is characterized by disproportionate dwarfism with very short extremities, which are bowed in type 1 (figure11)and maybe straight in type 2. The trunk length is normal, but the thorax is narrow. There is distinct flattening of vertebral ossification centers (platyspondyly) and a large head, depressed nasal bridge, prominent forehead, and protruding eyes. Skull deformity is often present due to the premature closure of cranial sutures. Cloverleaf skull deformity is generally seen in type 2. Polyhydramnios is present in almost 50% of cases.[29]
• Achondroplasia: is the most common nonlethal skeletal dysplasia without mental impairment. At around 20 weeks of gestation, fetuses with achondroplasia had normal biometric parameters, including FL, which became abnormally short only in the third trimester. The micromelia is rhizomelic, and the heads tend to be large. Typical facial features include a prominent forehead, depressed nasal bridge, and mid-face hypoplasia. The phalanges are short; typical gaps between the fingers and digital deviation lead to the appearance of a ‘trident’ hand.[30]
• Osteogenesis Imperfecta: a heterogeneous group of disorders caused by mutations in genes that encode type I collagen, leading to increased bone fragility. The major features are collapsed vertebral bodies, rib fractures, and, in more severe cases, thin shafts with fractures and bowing deformities. The fetal movements may be reduced. The skull may be thinner than usual, and the weight of the US probe may deform the head quite easily.[29]

7. Gender: If the external genitalia is to be visualized for gender determination, then imaging in both mid-sagittal and axial planes is recommended to minimize error. Firstly a mid-sagittal image of the lower abdomen below the cord insertion demonstrates penis and scrotum caudally to the cord insertion in a male fetus and the flat mons pubis caudal to the cord insertion in a female fetus. Secondly, a transverse image, just below the level of the bladder, best taken with the knees separated, demonstrates the penis and scrotum between the thighs in males and three lines representing the labia in females. Testes are undescended at this gestation and usually descend later in the third trimester.[19]

C. Placenta and amniotic fluid volume: It is important to assess the placental location, appearance, and its relation with the internal cervical os. Amniotic fluid volume can be estimated subjectively or by measuring the deepest vertical pocket. The latter of 2 cm or below and 8 cm or above is regarded as oligohydramnios and polyhydramnios, respectively.[14]

D. Cervix, uterine morphology, and adnexa: Currently, there is insufficient evidence to recommend routine screening of asymptomatic or symptomatic pregnant women with transvaginal ultrasound cervical length.[31] Maternal adnexal or uterine pathologies like mass that may interfere with fetal growth or labor should be documented.

Clinical Significance

The mid-trimester ultrasound scan should be performed according to international standards and preferably by accredited sonographers.

When a structural abnormality is detected, the questions the obstetrician would like to know are: how severe is the anomaly, is it lethal? Does the fetus have a chromosomal abnormality? What is the cause? How can the fetus be best salvaged? A fetal karyotype is indicated when there are multiple structural malformations, associated fetal growth retardation, certain anomalies like cystic hygroma, non-immune hydrops, and cardiovascular defects. It is better to know the fetal karyotype during pregnancy rather than to wait till after delivery, as quite often the baby may not survive, the chromosomal cultures might fail, and valuable information might be lost, which could be of vital importance for future counseling.

Management guidelines for a lethal anomaly like anencephaly and bilateral renal agenesis indicate the termination of pregnancy. If the defect is non-progressive and correctable at birth, as in mild hydronephrosis with good renal function, the decision would be to wait till term; delivery followed by neonatal treatment. Fetal therapy of a structurally malformed fetus requires a multidisciplinary team of the obstetrician, geneticist, neonatologist, and pediatric surgeon. Malformations that require correction in utero are those that would cause deterioration of the fetal organs if left alone.[3]

Enhancing Healthcare Team Outcomes

The optimal obstetric outcome can be expected by the early initiation of prenatal care. An essential element of prenatal care is the early detection of possible malformation, which is the primary goal of the mid-trimester anomaly scan. This cannot be achieved without optimized coordination amongst interprofessional team members, including well-trained sonographers, specialists -referral and parent counseling systems, nurses, obstetricians, fetal medicine specialists, surgeons, and others.