Sonography 2nd Trimester Assessment, Protocols, and Interpretation

Indications

An ultrasound scan performed between 18 and 22 weeks of gestation is appropriate in all pregnancies as a part of routine antenatal evaluation and screening because it provides the pregnant woman and her care provider with information about multiple aspects of her pregnancy. The obstetrical ultrasound will inform 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, detect soft markers of aneuploidy, and identify maternal pelvic pathology.[8]

Contraindications

Based on current evidence, routine clinical scanning of every woman during pregnancy using real-time B-mode imaging is not contraindicated. The American Institute of Ultrasound in Medicine (AIUM) and the 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 near a bone. This should not prevent using this mode when clinically indicated, provided the user has adequate knowledge of the instrument's acoustic output or 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 for entertainment purposes. The "as low as reasonably achievable" (ALARA) principle best expresses this unequivocal demand for safety.[9][10]

Equipment

The examination is mainly 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. Several factors influence the optimal transducer and operating frequency choice, including maternal habitus, fetal position, and the approach used. Most basic examinations are satisfactorily performed with 3 to 5 MHz transabdominal transducers.[11]

Personnel

The scan should be performed according to international standards and by accredited sonographers who have completed appropriate training programs supported by scientific societies.[11]

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 scan the patient to validate or change the differential diagnosis.[12]

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. This information is critical in correctly targeting specific structures, choosing a transvaginal and/or transabdominal technique, and determining whether additional studies may be helpful (eg, Doppler velocimetry).                                                 

Patient positioning: a semirecumbent position is most commonly used in obstetrics and gynecology. A padded table and pillow are used to ensure maximal comfort. It is preferable to elevate the head of the bed as some pregnant women cannot lie flat, especially in late pregnancy. For the transvaginal exam, a lithotomy position is used.                                                                                                                               

Special consideration for obese patients: in obese individuals, imaging can be improved by placing the transducer on the side rather than on the midline while the patient is lying on her side. The transvaginal probe is also helpful in these patients.[13]                                                                                              

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 falsely diagnose placenta previa or an elongated cervix. Transvaginal sonography is usually performed with an empty bladder.

Technique or Treatment

A. Fetal Biometry and Well-being: The following sonographic parameters can help to estimate gestational age and fetal size:

• 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 the 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.[14][15][16]

Measurements should be performed in a standardized manner based on strict quality criteria.[17] 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 estimating gestational age.[18][19]

B. Anatomical Survey: This comprehensive assessment encompasses the evaluation of various fetal structures and organs to ensure their normal development and functionality. To facilitate a clear understanding of the intricacies involved in this critical diagnostic procedure, the following table offers a systematic overview of the key anatomical elements typically examined during an anatomic survey (see Table. Minimal Requirements and Optional Views for the Routine Mid-Trimester Anomaly Scan).[16]

Table. Minimal Requirements and Optional Views for the Routine Mid-Trimester Anomaly Scan

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 (eg, lemon, strawberry, cloverleaf) should be documented and investigated.[20] 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 (eg, osteogenesis imperfecta, hypophosphatasia). Poor mineralization is also suggested when the skull becomes easily depressed due to manual pressure from transducer placement against the maternal abdominal wall.[21]                                                                                               
• Brain: The axial planes that allow the assessment of anatomic integrity include the following:
  • Transventricular plane: demonstrates the anterior and posterior portion of the lateral ventricles. The frontal horns (anterior portion) appear as 2 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 generally followed to rule out early ventriculomegaly.[11][22][11]
  • Transcerebellar 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 transcerebellar 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 the 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 mm to 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.[23]
  • Transthalamic plane: The anatomic landmarks include, from anterior to posterior, the frontal horns of the lateral ventricles, the CSP, the thalami, and the hippocampal gyruses.[24]

Examples of fetal head abnormalities:[25]

• Thickened nuchal skin fold: Assessment of the nuchal fold is typically made using a transcerebellar plane. A nuchal fold thickness of >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 an amniocentesis since the prevalence of cytogenetic abnormalities has been reported as high as 73%.
• Meningoceles are seen as fluid-filled cystic structures, and encephaloceles 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).[26]

Face/neck: early detection of facial abnormalities may lead to the diagnosis of multiple syndromes, which allows counseling of the parents and optimal care of the fetus. 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.[27]

Examples 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 approximately 40% to 80% of cases.[28]
• Hypertelorism and hypotelorism: the distance between the 2 inner canthi, known as interocular distance (IOD), approximates the ocular diameter (specific charts are used). The distance between the 2 outer canthi is 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 malformations.[25]
• Cleft lip and palate are the most common face malformations. Other congenital anomalies are seen in about half of the cases[25] 
• Neck masses include teratoma, lymphangioma, enlarged thyroid, branchial cleft cyst, and rarely a sarcoma.[25]

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 4-chamber view of the fetal heart should be obtained with the assessment of the following: the heart rate (from 120 to 160 beats 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), 2 atria, 2 ventricles, 2 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.[16]

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.[21] 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.[29]

Examples of fetal abdomen abnormalities:

• Echogenic bowel: defined as bowel with an echogenicity ≥ 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.[30]
• Fetal bowel dilatation: characterized by fluid-filled intestinal loops that 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.[31]
• 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 irregularly 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 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.[29]
• 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 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 earlier, and hepatic fibrosis is common in late-onset polycystic kidney disease. Perinatal mortality is usually caused by pulmonary hypoplasia after severe oligo- or anhydramnios.[29]
• When evaluating urinary bladder distention and pyelectasis in the male fetus, 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 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.[32]
• The 2 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 is characterized by a full-thickness abdominal wall closure defect, which permits evisceration of the fetal abdominal contents, specifically bowel herniation. 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 due to 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%).[33]

Spine: The choice of the scanning planes used to evaluate the spine's integrity 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 2 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 to 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.[11]

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 critical determinations that ultrasound must make is neonatal or infantile lethality. Lethality occurs in most skeletal dysplasias due to a small chest circumference and resultant pulmonary hypoplasia. Using ultrasound criteria for lethality, a chest-to-abdominal circumference ratio of <0.6 and a femur length-to-abdominal circumference ratio of <0.16 strongly suggest lethality.[34]

Examples of fetal skeletal dysplasia:

• Thanatophoric Dysplasia: is the most common lethal skeletal dysplasia. Inheritance is generally autosomal dominant. It is characterized by disproportionate dwarfism with very short extremities, which are bowed in type 1 and maybe straight in type 2. The trunk length is normal, but the thorax is narrow. There is a 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.[35]
• Achondroplasia: is the most common nonlethal skeletal dysplasia without mental impairment. At about 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 head tends to be large. Typical facial features include a prominent forehead, depressed nasal bridge, and mid-face hypoplasia. The phalanges are short; typical gaps are seen between the fingers, and digital deviation leads to the appearance of a " trident'’ hand.[36] 
• Osteogenesis Imperfe ista: a heterogeneous group of disorders caused by gene mutations that encode type I collagen, increasing bone fragility. The major features are collapsed vertebral bodies, rib fractures, and, in more severe cases, thin shafts with fractures and bowing deformities. 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.[35]

Gender: If the external genitalia is to be visualized for gender determination, imaging in both mid-sagittal and axial planes is recommended to minimize error. A mid-sagittal image of the lower abdomen below the cord insertion demonstrates the 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. 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 3 lines representing the labia in females. Testes are undescended at this gestation and usually descend later in the third trimester.[23]

C. Placenta and Amniotic Fluid Volume: assessing the placental location, appearance, and its relation with the internal cervical os is essential. Amniotic fluid volume can be estimated subjectively or by measuring the deepest vertical pocket. A vertical pocket of ≤2 cm and ≥8 cm are regarded as oligohydramnios and polyhydramnios, respectively.[16][37][16]

D. Cervix, Uterine Morphology, and Adnexa: There is insufficient evidence to recommend routine screening of asymptomatic and symptomatic pregnant women with transvaginal ultrasound cervical length.[38] Maternal adnexal or uterine pathologies like masses 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 obstetrician wants to know: 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 wait until after delivery, as the baby may not often survive, the chromosomal cultures might fail, and valuable information might be lost, which could be vital for future counseling.

Management guidelines for a lethal anomaly like anencephaly and bilateral renal agenesis indicate termination of the pregnancy. If the defect is nonprogressive and correctable at birth, as in mild hydronephrosis with good renal function, the decision would be to wait until term delivery followed by neonatal treatment. Fetal therapy of a structurally malformed fetus requires a multidisciplinary team comprised 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.[4]

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 malformations, which is the primary goal of the mid-trimester anomaly scan. This cannot be achieved without optimized coordination among interprofessional team members, including well-trained sonographers, specialists-referral and parent counseling systems, nurses, obstetricians, fetal medicine specialists, surgeons, and others.

Health professionals involved in second-trimester obstetric sonography must possess specific technical skills, including proficiency in operating ultrasound equipment, identifying fetal structures, and interpreting sonographic images accurately. Continuous training and skill development are essential to maintain competency in this rapidly evolving field.

A well-defined strategy involves establishing standardized protocols and guidelines for conducting second-trimester obstetric sonography. This ensures consistency in care delivery and reduces variations in practice. It also includes setting objectives for the scan, such as screening for fetal anomalies or assessing fetal growth based on the patient's specific clinical indications.

Effective interprofessional communication is vital for the success of obstetric sonography. Physicians, nurses, and sonographers must collaborate closely to ensure that the scan is performed correctly, that findings are communicated accurately, and that patients receive clear explanations. Open and respectful communication among team members fosters trust and enhances patient care.

Ultimately, the interprofessional team can enhance patient-centered care in second-trimester obstetric sonography by fostering a collaborative approach that emphasizes skills development, shared responsibilities, effective communication, and care coordination. This leads to improved patient outcomes, safety, and team performance while respecting the unique challenges and complexities of obstetric care.