Practice guidelines for performance of the routine mid‐trimester fetal ultrasound scan

The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) is a scientific organization that encourages sound clinical practice, teaching and research for diagnostic imaging in women's healthcare. The ISUOG Clinical Standards Committee (CSC) has a remit to develop Practice Guidelines and Consensus Statements as educational recommendations that provide healthcare practitioners with a consensus-based approach for diagnostic imaging. They are intended to reflect what is considered by ISUOG to be the best practices at the time at which they were issued. Although ISUOG has made every effort to ensure that guidelines are accurate when issued, neither the Society nor any of its employees or members accepts any liability for the consequences of any inaccurate or misleading data, opinions or statements issued by the CSC. They are not intended to establish a legal standard of care because interpretation of the evidence that underpins the guidelines may be influenced by individual circumstances and available resources. Approved guidelines can be distributed freely with the permission of ISUOG (email protected). Ultrasonography is widely used for the prenatal evaluation of growth and anatomy as well as for the management of multiple gestations. The procedure provides diagnostic findings that often facilitate the management of problems arising in later pregnancy. For example, abnormal fetal growth is a leading cause of perinatal morbidity and mortality in both industrialized and developing countries. In 2005, the World Health Organization (WHO) concluded that impaired fetal growth had many causes related to: genetic factors, maternal characteristics such as nutrition, lifestyle including smoking, age and disease; complications of pregnancy; and the physical, social and economic environment1, 2. A mid-trimester fetal ultrasound scan serves as an important baseline against which later scans may be compared for the evaluation of growth and health. Ultrasonography can also be used to detect congenital anomalies3-6. The Eurofetus study7, a multicenter project involving 61 obstetric ultrasound units from 14 European countries, examined the accuracy of routine mid-trimester ultrasonographic examination in unselected populations. Over one half (56%) of 4615 malformations were detected and 55% of major anomalies were identified before 24 weeks of gestation. Although many countries have developed local guidelines for the practice of fetal ultrasonography, there are still many areas of the world where they have not been implemented. Most countries offer at least one mid-trimester scan as part of standard prenatal care, although obstetric practice varies widely around the world. This can be related to the availability of qualified practitioners and equipment, local medical practice and legal considerations; in some countries, insurance-related cost reimbursements strongly influence how routine mid-trimester scans are implemented. Nonetheless, a WHO Study Group stated: ‘Worldwide, it is likely that much of the ultrasonography currently performed is carried out by individuals with in fact little or no formal training.’8. The intent of this document is to provide further guidance for healthcare practitioners in the performance of the mid-trimester fetal ultrasound scan. The main objective of a routine mid-trimester fetal ultrasound scan is to provide accurate diagnostic information for the delivery of optimized antenatal care with the best possible outcomes for mother and fetus. The procedure is used to determine gestational age and to perform fetal measurements for the timely detection of growth abnormalities later in pregnancy. Other goals are to detect congenital malformations and multiple pregnancies. - cardiac activity; - fetal number (and chorionicity if multiple pregnancy); - fetal age/size; - basic fetal anatomy; - placental appearance and location. Although many malformations can be identified, it is acknowledged that some may be missed, even with sonographic equipment in the best of hands, or that they may develop later in pregnancy. Before starting the examination, a healthcare practitioner should counsel the woman/couple regarding the potential benefits and limitations of a routine mid-trimester fetal ultrasound scan. Many countries offer at least one routine mid-trimester fetal ultrasound scan. As one example, an imaging workshop organized by the Eunice Kennedy Shriver National Institute of Child Health and Human Development in the United States9 reached a consensus that all pregnant women should be offered an ultrasound scan for the detection of fetal anomalies and pregnancy complications. Serial scans may be helpful for some mothers with risk factors for adverse pregnancy outcome (e.g. hypertension or diabetes) and others may benefit from more detailed scans that are targeted to their specific situation. Repeated or detailed examinations, however, are not considered to be routine scans. A routine mid-trimester ultrasound scan is often performed between 18 and 22 weeks of gestation. This period represents a compromise between dating the pregnancy (more accurate if established earlier) and the timely detection of major congenital anomalies. Countries where pregnancy termination is restricted should balance detection rates against the time needed for counseling and additional investigation. Some centers perform the anatomical survey using transvaginal scanning at approximately 13–16 weeks' gestation. This earlier approach can provide useful information about gestational age as a baseline for growth assessment or determination of chorionicity for twins, but may require special training for the early interpretation of anatomical structures. Individuals who routinely perform obstetric scans should have specialized training for the practice of diagnostic ultrasonography in pregnant women. However, the requirements for this activity may vary depending on the country. - trained in the use of diagnostic ultrasonography and related safety issues; - regularly perform fetal ultrasound scans; - participate in continuing medical education activities; - have established appropriate referral patterns for suspicious or abnormal findings; - routinely undertake quality assurance and control measures. - real time, gray-scale ultrasound capabilities; - transabdominal transducers (3–5-MHz range); - adjustable acoustic power output controls with output display standards; - freeze frame capabilities; - electronic calipers; - capacity to print/store images; - regular maintenance and servicing, important for optimal equipment performance. An examination report should be produced as an electronic and/or a paper document, to be sent to the referring care provider in reasonable time. A sample reporting form is available at the end of this article. Images of standard views (stored either electronically or as printed copies) should also be produced and stored. Motion videoclips are recommended for the fetal heart. Local laws should be followed. Many jurisdictions require image storage for a defined period of time. Prenatal ultrasonography appears to be safe for clinical practice. To date, there has been no independently confirmed study to suggest otherwise. Fetal exposure times should be minimized, using the lowest possible power output needed to obtain diagnostic information, following the ALARA principle (As Low As Reasonably Achievable). More details are available from the ISUOG Safety Statement11. These recommendations represent minimum practice guidelines for the mid-trimester fetal ultrasound scan. Consideration must be given to local circumstances and medical practices. Reasons for deviations from these recommendations should be documented. If the examination cannot be performed completely in accordance with adopted guidelines, the scan should be repeated, at least in part, at a later time, or the patient can be referred to another practitioner. This should be done as soon as possible, to minimize unnecessary patient anxiety and unnecessary delay in the potential diagnosis of congenital anomalies or growth disturbances. Individuals who perform ultrasonographic scans during pregnancy should have referral mechanisms in place to manage suspected or detected abnormalities. A minimum examination, following the guidelines presented herein, should be performed before referring the patient, unless technical factors prevent completion of the initial evaluation. - biparietal diameter (BPD); - head circumference (HC); - abdominal circumference (AC) or diameter; - femur diaphysis length (FDL). Measurements should be performed in a standardized manner on the basis of strict quality criteria15. An audit of results can help to ensure accuracy of techniques with regard to specific reference tables. An image(s) should be taken to document the measurement(s). Examples of still images appropriate for fetal biometry are demonstrated in Figure 1. Standard fetal biometry: sonographic measurements of the biparietal diameter and head circumference (a), the abdominal circumference (b) and the femur diaphysis length (c). In this example, calipers are placed on the outer and inner edges of the skull for BPD measurement (large white dots in (a)); some reference charts have been developed using different caliper placement for this measurement (e.g. outer edge to outer edge of the skull). If gestational age has not already been established at a dating or first-trimester scan, it should be determined at the mid-trimester scan on the basis of fetal head size (BPD and/or HC) or FDL. The chosen reference standards should be indicated in the report16. Subsequent scans should not be used to calculate a new estimated date of confinement if age has already been established by a high-quality scan earlier in the pregnancy. Additional measurements, optimally at least 3 weeks from a preceding scan, are usually reported as deviations from mean values with their expected ranges for a given age. This information can be expressed as Z-scores, percentile reference ranges or on a graph, although the degree of deviation from normal at this early stage of pregnancy that would justify action (e.g. a follow-up scan to assess fetal growth or fetal chromosomal analysis) has not been firmly established. Combining measurements significantly improves accuracy compared with prediction based on HC alone17. However, the clinical significance of this improvement is marginal because the improved accuracy represents less than 1 day18. - Cross-sectional view of the fetal head at the level of the thalami; - ideal angle of insonation is 90° to the midline echoes; - symmetrical appearance of both hemispheres; - continuous midline echo (falx cerebri) broken in middle by the cavum septi pellucidi and thalamus; - no cerebellum visualized. Both calipers should be placed according to a specific methodology, because more than one technique has been described (e.g. outer edge to inner edge or ‘leading edge’ technique vs. outer edge to outer edge), at the widest part of the skull, using an angle that is perpendicular to the midline falx (Figure 1)19. The same technique as that used to establish the reference chart should be used. The cephalic index is a ratio of the maximum head width to its maximum length and this value can be used to characterize fetal head shape. Abnormal head shape (e.g. brachycephaly and dolichocephaly) can be associated with syndromes. This finding can also lead to inaccurate estimates of fetal age when the BPD is used; in these cases, HC measurements are more reliable20. As described for the BPD, ensuring that the circumference placement markers correspond to the technique described on the reference chart. If the ultrasound equipment has ellipse measurement capacity, then the HC can be measured directly by placing the ellipse around the outside of the skull bone echoes (Figure 1). Alternatively, the HC can be calculated from the BPD and occipitofrontal diameter (OFD) as follows: the BPD is measured using a leading edge technique as described in the previous section whereas the OFD is obtained by placing the calipers in the middle of the bone echo at both the frontal and occipital skull bones. HC is then calculated using the equation: HC = 1.62× (BPD + OFD). - Transverse section of the fetal abdomen (as circular as possible); - umbilical vein at the level of the portal sinus; - stomach bubble visualized; - kidneys should not be visible. The AC is measured at the outer surface of the skin line, either directly with ellipse calipers or calculated from linear measurements made perpendicular to each other, usually the anteroposterior abdominal diameter (APAD) and transverse abdominal diameter (TAD) (Figure 1). To measure the APAD, the calipers are placed on the outer borders of the body outline, from the posterior aspect (skin covering the spine) to the anterior abdominal wall. To measure the TAD, the calipers are placed on the outer borders of the body outline, across the abdomen at the widest point. The AC is then calculated using the formula: AC = π (APAD + TAD)/2 = 1.57 (APAD + TAD). The FDL is imaged optimally with both ends of the ossified metaphysis clearly visible21, 22. The longest axis of the ossified diaphysis is measured. The same technique as that used to establish the reference chart should be used with regard to the angle between the femur and the insonating ultrasound beams. An angle of insonation between 45° and 90° is typical. Each caliper is placed at the ends of the ossified diaphysis without including the distal femoral epiphysis if it is visible (Figure 1). This measurement should exclude triangular spur artifacts that can falsely extend the diaphysis length. Mid-trimester sonographic measurements can be used to identify abnormalities of fetal size23, 24. Some countries also use this information to estimate fetal weight as a baseline parameter for the detection of subsequent growth problems. Many ‘size discrepancies’ are explained by incorrect menstrual age estimates, even in women with ‘certain dates’25, 26. If gestational age is determined at an earlier scan, EFW can be compared to dedicated normal, preferably local, reference ranges for this parameter14, 27, 28. However, the degree of deviation from normal at this early stage of pregnancy that would justify action (e.g. follow-up scan to assess fetal growth or fetal chromosomal analysis) has not been firmly established. Amniotic fluid volume can be estimated subjectively or using sonographic measurements. Subjective estimation is not inferior to the quantitative measurement techniques (e.g. deepest pocket, amniotic fluid index) when performed by experienced examiners29, 30. Patients with deviations from normal should have more detailed anatomical evaluation and clinical follow-up. Normal fetuses typically have a relaxed position and show regular movements. There are no specific movement patterns at this stage of pregnancy. Temporary absence or reduction of fetal movements during the scan should not be considered as a risk factor31. Abnormal positioning or unusually restricted or persistently absent fetal movements may suggest abnormal fetal conditions such as arthrogryposis32. The biophysical profile is not considered part of a routine mid-trimester scan33. The application of Doppler techniques is not currently recommended as part of the routine second-trimester ultrasound examination. There is insufficient evidence to support universal use of uterine or umbilical artery Doppler evaluation for the screening of low-risk pregnancies34-36. - visualization of the placental cord insertion; - distinguishing features (gender, unique markers, position in uterus); - determination of chorionicity is sometimes feasible in the second trimester if there are clearly two separate placental masses and discordant genders. Chorionicity is much better evaluated before 14–15 weeks (lambda sign or T-sign). Follow-up of multiple pregnancies should be arranged in accordance with local guidelines and clinical practices. Recommended minimum requirements for a basic fetal anatomical survey during the mid-trimester of pregnancy are summarized in Table 1. - Size: measurements are performed as mentioned in the biometry section. - Shape: the skull normally has an oval shape without focal protrusions or defects and is interrupted only by narrow echolucent sutures. Alterations of shape (e.g. lemon, strawberry, cloverleaf) should be documented and investigated41. - Integrity: no bony defects should be present. Rarely, brain tissue can extrude through defects of the frontal or occipital bones, although cephaloceles may occur at other sites as well. - Density: normal skull density is manifested as a continuous echogenic structure that is 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)42. 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. Transverse views of the fetal head demonstrating standard transventricular (a), transthalamic (b) and transcerebellar (c) scanning planes. The first two planes allow assessment of the anatomical integrity of the brain. The third permits evaluation of the cerebellum and cisterna magna in the posterior fossa. - lateral ventricles (including choroid plexi); - cavum septi pellucidi; - midline falx; - thalami; - cerebellum; - cisterna magna. Minimum evaluation of the fetal face should include an attempt to visualize the upper lip for possible cleft lip anomaly43 (Figure 3a). If technically feasible, other facial features that can be assessed include the median facial profile (Figure 3b), orbits (Figure 3c), nose and nostrils. Ultrasound imaging of the fetal face. The mouth, lips and nose are typically evaluated in a coronal view (a). If technically feasible, a median facial profile provides important diagnostic clues for cleft lip, frontal bossing, micrognathia and nasal bone anomalies (b). Both fetal orbits should appear symmetrical and intact (c). The neck normally appears as cylindrical with no protuberances, masses or fluid collections44. Obvious neck masses such as cystic hygromas or teratomas should be documented. The shape should be regular with a smooth transition to the abdomen45. The ribs should have normal curvature without deformities. Both lungs should appear homogeneous and without evidence of mediastinal shift or masses. The diaphragmatic interface can often be visualized as a hypoechoic dividing line between the thoracic and abdominal content (e.g. liver and stomach)46, 47. The basic and extended basic cardiac ultrasonographic examinations are designed to maximize the detection of congenital heart disease during a second-trimester scan (Figure 4)48. A single acoustic focal zone and relatively narrow field of view can help to maximize frame rates. Images should be magnified until the heart fills at least a third to half of the display screen. Basic and extended basic views of the fetal heart. The basic cardiac scan is obtained from a four-chamber view (a) when both ventricles are seen during end diastole (calipers). An extended basic scan of the great arteries demonstrates the left (b) and right (c) ventricular outflow tracts. Separate arterial outflow tracts (calipers), approximately equal in size, exit their respective ventricles by crossing over each other in normal fetuses. The basic cardiac screening examination is interpreted from a four-chamber view of the fetal heart. A normal regular rate ranges from 120 to 160 beats per min. The heart should be located in the left chest (same side as the fetal stomach) if the situs is normal. A normal heart is usually no larger than one-third of the area of the chest and is without pericardial effusion. The heart is normally deviated by about 45 ± 20° (2 SD) towards the left side of the An extended basic cardiac which the and outflow can the detection rates for major cardiac malformations by the four-chamber view additional to of the basic examination are more likely to identify anomalies such as of of the great right and Normal great are approximately equal in size and should each other as they exit from their respective ventricular Some have described an and that may also be useful for the and right in of their and anatomical For a more detailed of fetal cardiac the is referred to the ISUOG guidelines for the fetal cardiac examination. This document can be from the situs should be The fetal stomach should be identified in its normal position on the left should be the abdomen and the umbilical cord should an intact abdominal wall. Abnormal fluid of the (e.g. should be documented. from the a fetal may be seen in the right upper to the although this finding is not a minimum of the basic scan. other cystic seen in the abdomen should referral for a more detailed scan. The fetal umbilical cord (Figure should be examined for evidence of a such as or may also be using gray-scale imaging as an of the routine anatomical Ultrasound imaging of the fetal cord with umbilical kidneys and The umbilical cord the fetal abdomen provides information about the of defects such as or The fetal and both kidneys should be and views of the provide screening for when these scanning planes are abnormal in the of frontal skull and an cisterna magna The fetal and both kidneys should be identified and If either or appears a measurement should be documented. to visualize the should referral for a more detailed A examination of the fetal and and the results are fetal position and evaluation of the fetal from every is not part of the basic examination, although transverse and views are usually The of the is usually associated with abnormal anatomy such as a and cisterna magna. Other views of the fetal may identify other including abnormalities and The or absence of both (Figure and both (Figure should be documented using a or is not as part of the routine mid-trimester scan. of the fetal upper and The or absence of the upper and should be documented routinely unless they are visualized to technical factors position should be determined in to the maternal (c). ultrasonography, the placental (Figure its with the and its appearance should be Examples of abnormal placental findings include the of multiple with and placental masses such as In of the routine second-trimester examination, transabdominal ultrasonography permits of the between and If the placental edge or the a follow-up examination in the third trimester is with a of uterine and anterior or are at risk for placental In these cases, the should be examined for findings of the of which are the of multiple placental that show arterial or Abnormal appearance of the uterine interface is specific for but is seen in of the echolucent between an anterior and the uterine is neither a nor a specific for Although may be suspected during a routine mid-trimester scan, a more detailed evaluation is usually to further this of to determine fetal is not considered in the of a mid-trimester routine scan. of should be considered only with and in the of local practices. have demonstrated a between length on transvaginal scan and subsequent However, that examined the of routine length measurement and subsequent to any of such screening there is insufficient evidence to routine length measurements at the trimester in an unselected of women with length may have benefits for research and further but this is not a for routine a universal screening would not only require and quality but also cause potential by anxiety and unnecessary and masses should be documented if they are likely to with These guidelines were developed by the Prenatal Ultrasound the of the ISUOG Clinical Standards of Obstetrics and of is extended to who to this of and of of Obstetrics and of Obstetrics and The of Fetal National Institute of of of Fetal of Obstetrics and The of Fetal of Obstetrics and of Obstetrics and of Obstetrics and of of of to for to the Safety section and to for some of the of this document are available ISUOG