The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) is a scientific organization that encourages sound clinical practice, and high-quality teaching and research related to 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, from experts, for diagnostic imaging. They are intended to reflect what is considered by ISUOG to be the best practice at the time at which they are 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 liability for the consequences of any inaccurate or misleading data, opinions or statements issued by the CSC. The ISUOG CSC documents 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, local protocol and available resources. Approved Guidelines can be distributed freely with the permission of ISUOG (info@isuog.org). Ultrasonography is used widely for the prenatal evaluation of fetal growth and anatomy, as well as for the management of multiple gestations. The mid-trimester ultrasound scan is performed mainly for anatomical evaluation of the fetus. In experts' hands, most clinically important structural anomalies can be detected1. However, there are significant differences in detection rates between centers and between operators. The mid-trimester fetal ultrasound scan also serves as a baseline against which later scans can be compared for the evaluation of fetal growth. 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 one mid-trimester scan as part of routine prenatal care. This document, which constitutes an updated version of previously published guidelines2, suggests the standards that this scan should aim to achieve. Details of the grades of recommendation and levels of evidence used in ISUOG Guidelines are given in Appendix 1. 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. In some settings, measurement of cervical length (CL) is offered to women at the time of the mid-trimester scan within the context of prediction and prevention of preterm birth. A current suggestion is that CL measurements should be done by transvaginal scanning, which requires additional consent from the woman, appropriate training of the operator3 and auditing of the results. When CL measurement can be carried out meeting these conditions, it can be considered as an integral part of the routine mid-trimester scan. The ‘ISUOG Practice Guidelines: role of ultrasound in the prediction of spontaneous preterm birth’ (in prep.) will provide more guidance and details. When uterine and adnexal masses (fibroids, ovarian cysts) are visualized, they should be reported, but formal assessment of uterine and adnexal anatomy is not part of the routine mid-trimester scan. Although many fetal malformations and anomalies can be identified at this mid-trimester scan, some may be missed or may become apparent only later in pregnancy, even with the best sonographic equipment in the best of hands. These Guidelines refer to routine ultrasound evaluation of pregnant women who have no maternal, fetal or obstetric risk factors. Even if risk factors are present, it is still appropriate to consider a mid-trimester scan following these Guidelines, for baseline pregnancy evaluation. Additional, more comprehensive, detailed ultrasonographic examinations in response to specific clinical situations should be performed to address specific needs. These are best performed by specialists experienced in such comprehensive evaluations, and are beyond the scope of these general Guidelines. Individuals or clinics performing routine ultrasonographic scans during pregnancy should have referral mechanisms in place to manage suspected or detected anomalies. A complete screening examination according to the Guidelines presented here should still be performed before referring a woman, unless technical factors prevent completion of the initial evaluation. Measurements should be performed in a standardized manner on the basis of strict quality criteria15-17 and in accordance with ISUOG Practice Guidelines13. An image should be obtained to document each measurement. Examples of still images appropriate for fetal biometry are demonstrated in Figure 1. An audit of results can help to ensure accuracy of techniques with regard to specific reference tables16, 18. A first-trimester ultrasound examination should have been offered routinely4, allowing exact gestational-age assessment. If gestational age has not already been established at a dating or first-trimester scan, it should be determined at the mid-trimester scan. Although head measurements (BPD and HC) and FL have all been used in the past, recent evidence from the INTERGROWTH-21st study indicates that HC alone or HC plus FL may be the most accurate predictor of gestational age after 14 weeks19. Subsequent scans should not be used to calculate a new estimated date of confinement if gestational age has already been established by a high-quality scan earlier in the pregnancy. Caliper placement. Both calipers should be placed according to a specific methodology, because more than one technique has been described (e.g. outer-to-inner edge (‘leading edge’ technique) vs outer-to-outer edge), at the widest part of the skull, perpendicular to the midline. 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 (BPD) to its maximum length (occipitofrontal diameter (OFD)) and this value can be used to characterize the fetal head shape. Abnormal head shape (e.g. brachycephaly or dolichocephaly) can be associated with syndromes or be the result of oligohydramnios or breech presentation. This finding can also lead to inaccurate estimates of fetal age when the BPD is used; in these cases, HC measurements are even more reliable20, 21. Recent evidence suggests that outer-to-outer placement of calipers eases standardization, reproducibility and quality control22. Caliper placement. As for the BPD, it is important to ensure that the HC placement markers correspond to those used for the reference chart. If the ultrasound equipment has ellipse measurement capacity, the HC can be measured directly by placing the ellipse around the outside of the skull bone echoes (Figure 1a). Alternatively, the HC can be calculated from the BPD and OFD as follows: the BPD is measured using a leading-edge technique, as described in the ‘Biparietal diameter’ section, above, 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 as HC = 1.62 × (BPD + OFD). Recent evidence suggests that outer-to-outer placement of calipers eases standardization, reproducibility and quality control22. Caliper placement. The AC is either measured directly at the outer surface of the skin line, with ellipse calipers (Figure 1b), or calculated from linear measurements made perpendicular to each other, usually the anteroposterior abdominal diameter (APAD) and the transverse abdominal diameter (TAD). 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 as AC = 1.57 × (APAD + TAD). Anatomy. The FL is imaged with both ends of the ossified diaphysis visible. 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 beam. An angle of insonation between 45° and 90° is typical. Technical improvements in modern ultrasound machines have reduced the beam width, which has affected fetal measurements in the lateral direction23. This has clinical implications and recent measurement charts should be used, as using older ones may lead to an overestimation of the FL24. Caliper placement. Each caliper is placed at the ends of the ossified diaphysis without including the distal femoral epiphysis if it is visible (Figure 1c). This measurement should exclude triangular spur artifacts that can extend the diaphysis length falsely. Estimated fetal weight (EFW) Despite many efforts to develop new models for calculating EFW, the three-parameters (HC, AC, FL) formula reported by Hadlock et al.25 provided the best fetal weight estimates in a large study cohort27, and should be considered the method of choice for assessment of all fetuses, including those suspected to be either small or large13. Various approaches may be used to optimize the detection of abnormal growth14. 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 established. Recent research suggests that EFW as early as the mid trimester could be used in a competing-risks model to predict subsequent small-for-gestational age28. Additional measurements to demonstrate evidence of growth, taken at least 3 weeks from those obtained at a preceding scan, are usually reported as deviations from mean values with their expected ranges for a given age29. This information should preferentially be expressed as percentile of a reference range or Z-score, or on a graph. The use of Z-scores allows monitoring of severe anomalies and facilitates data quality control. The chosen reference standards should be indicated in the report30, 31. Fetal biometry charts which are prescriptive, obtained prospectively, truly population-based and derived from studies with the lowest possible methodological bias should be favored, although practitioners should be aware of nationally or locally recommended charts13. Whenever abnormal growth is suspected, the use of diagnostic criteria for fetal growth restriction (FGR) based on the Delphi 2016 consensus criteria should be encouraged13, 14, 32, 33. Abnormal umbilical artery Doppler indices and/or maternal symptoms of hypertension or pre-eclampsia should prompt emergency referral. The amount of amniotic fluid should be evaluated either subjectively, defined as ‘normal’ or ‘abnormal’ (reduced or increased), or semiquantitatively, by measurement of the deepest vertical pocket (DVP) of amniotic fluid or the amniotic fluid index (AFI). For DVP, the largest vertical pocket free of umbilical cord or fetal parts is measured. DVP ≤ 2.0 cm is considered as decreased amniotic fluid volume, DVP > 2 cm and ≤ 8.0 cm as normal amniotic fluid volume, and DVP > 8 cm as increased amniotic fluid volume34. Reference values for gestational age can also be used35. From 16 weeks onwards, the amnion and chorion are usually fused. Amniotic sheets are benign findings, to be distinguished from amniotic bands which may cause fetal deformities40-42. Normal fetuses typically have a neutral position and show regular movements. Temporary absence of or a reduction in fetal movements during the scan should not be considered as a risk factor43. Abnormal positioning or unusually restricted or persistently absent fetal movements may suggest abnormal fetal conditions, such as arthrogryposis, and should prompt a request for referral44. The biophysical profile is not considered part of the routine mid-trimester scan45. The insertion of the umbilical cord is in the center of the placenta in about 80% of cases, paracentral in about 12% of cases and marginal (within 2 cm of the placental edge) in 5–8% of cases. Velamentous insertion occurs in approximately 1% of cases, and is defined as insertion of the umbilical vessels within the amniotic membranes instead of the placenta46. A velamentous cord insertion may be associated with vasa previa and FGR. When marginal or velamentous insertion is visualized, it should be reported; however, formal assessment of umbilical cord insertion on the placenta is not part of the routine mid-trimester scan47. Number of vessels. Single umbilical artery (SUA) is the result of obliteration or atrophy of one of the arteries, most commonly the left48. It is more frequent in twin pregnancy. The diagnosis is made by direct visualization of the umbilical cord, or by tracking the umbilical arteries around the fetal bladder with color Doppler. SUA is associated with congenital anomalies and FGR49, although it does not constitute an anomaly per se. Therefore, care should be taken not to cause anxiety to the parents if no major anomaly is found at the mid-trimester scan. There is, as yet, no consensus regarding the potential impact of SUA on pregnancy outcome50, 51. Coiling. Coiling describes the spiral course of the umbilical arteries in the cord. Increased or reduced umbilical cord coiling have no proven significance and should not be reported as part of the routine mid-trimester scan52. When no first-trimester ultrasound examination has been performed and it is not possible to identify two separate placentae and the fetal gender is the same, the pregnancy should be considered as monochorionic and referred or followed as a high-risk pregnancy. Local guidelines and clinical practice should be followed. Suggested minimum requirements for a basic fetal anatomical survey during the mid trimester of pregnancy are summarized in Table 1. If any anomaly is suspected, then a more detailed examination or referral to an expert center should be considered. Intact cranium Head shape normal Cavum septi pellucidi normal in appearance Choroid plexus normal in appearance Midline falx normal in appearance Thalami normal in appearance Lateral cerebral ventricles normal in appearance Cerebellum normal in appearance Cisterna magna normal in appearance Nuchal fold** Optional component of checklist: can be evaluated if technically feasible and according to local practice. normal in appearance Both orbits and bulbi present Midsagittal facial profile** Optional component of checklist: can be evaluated if technically feasible and according to local practice. normal in appearance Nasal bone** Optional component of checklist: can be evaluated if technically feasible and according to local practice. normal in appearance Upper lip intact Chest and lungs appearing normal in shape/size Heart activity present Four-chamber view of heart in normal position (left chambers on left side) Aortic and pulmonary outflow tracts (relative size and their relationships) normal LVOT view; three-vessel view or three-vessels-and-trachea view normal No evidence of diaphragmatic hernia Stomach in normal position on left side Bowel normal (not dilated or hyperechogenic) Gallbladder on right side** Optional component of checklist: can be evaluated if technically feasible and according to local practice. Both kidneys present, no pyelectasis Urinary bladder normal in appearance Cord insertion site into the fetal abdomen normal No spinal defects or masses (transverse and sagittal views) Arms and hands present, normal joint position Legs and feet present, normal joint position Placental position and relation to cervix normal No masses present Three-vessel cord** Optional component of checklist: can be evaluated if technically feasible and according to local practice. Cord insertion into placenta** Optional component of checklist: can be evaluated if technically feasible and according to local practice. normal The scanning procedure should begin with a four-chamber view of the fetal heart. A normal, regular heart rate typically ranges from 120 to 160 bpm. The heart is positioned in the left chest (as is 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 axis deviates by approximately 45 ± 20° (2 SD) towards the left side of the fetus72. Routine cardiac screening should also assess the aortic and pulmonary outflow tracts to detect cardiac malformations beyond those achievable using the four-chamber view alone (Figure 4a). Normal-appearing great vessels are approximately equal in size and should cross each other as they exit their respective ventricular chambers (Figure 4b,c). Routine assessment of the cardiac outflow tracts in addition to the four-chamber view increases the screening performance for identifying conotruncal anomalies, such as tetralogy of Fallot, transposition of the great arteries, double-outlet right ventricle and truncus arteriosus communis. The three-vessel view and closely related three-vessels-and-trachea view may improve detection of outflow tract, aortic arch and systemic vein anomalies (Figure 4d,e)73-77. For a more detailed description of fetal cardiac screening, please refer to the ISUOG Guidelines for the fetal cardiac examination71. Vasa previa, defined as unprotected fetal vessels running through the fetal membranes, over or within 2 cm of the internal cervical os, is found in approximately 0.5 per 1000 pregnancies in the general population. Risk factors for vasa previa include twin pregnancy, conception by assisted reproductive technology, a low-lying or bilobed placenta, succenturiate placental lobes and velamentous cord insertion95. If such risk factors are identified, a targeted examination is suggested, given that prenatal knowledge of vasa previa significantly increases survival and decreases perinatal morbidity96. This can be done using a transvaginal approach with color Doppler imaging88, 97, 98. Similarly, when the transabdominal scan suggests the possibility of placenta previa or shortened/dilated maternal cervix, using transvaginal sonography with color Doppler imaging may also be of benefit. There is, however, ongoing debate regarding whether routine screening for velamentous cord insertion and/or vasa previa should be performed at the mid-trimester scan; the evidence is of limited quality and fails to take into account the consequences of over-diagnosing such anomalies47, 88. Furthermore, not all medical practices may have sufficient experience in transvaginal sonography or the resources for proper disinfection procedures. In women with singleton gestation, a short cervix and prior spontaneous preterm birth, cerclage is associated with significant decrease in the risk of preterm birth and neonatal morbidity and mortality107. Several medical societies recommend serial transvaginal sonographic CL measurement at 16–23 weeks in this population104, 105, 108, 109. The ‘ISUOG Practice Guidelines: role of ultrasound in the prediction of spontaneous preterm birth’ (in prep.) will provide more guidance and details. L. J. Salomon, Department of Obstetrics and Fetal Medicine, Hôpital Necker-Enfants Malades, Assistance Publique-Hopitaux de Paris, Paris Cité University, Paris, France Z. Alfirevic, Department of Women's and Children's Health, University of Liverpool, Liverpool, UK V. Berghella, Thomas Jefferson University, Obstetrics and Gynecology, Division of Maternal Fetal Medicine, Philadelphia, PA, USA C. M. Bilardo, University Medical Centre, Fetal Medicine Unit, Department of Obstetrics & Gynecology, Groningen, The Netherlands G. E. Chalouhi, Maternité Necker-Enfants Malades, Université Paris Descartes, AP-HP, Paris, France F. Da Silva Costa, Maternal Fetal Medicine Unit, Gold Coast University Hospital and School of Medicine, Griffith University, Gold Coast, Queensland, Australia E. Hernandez-Andrade, University of Texas Health Science Center at Houston, Houston, TX, USA G. Malinger, Division of Ob-Gyn Ultrasound, Lis Maternity Hospital, Tel Aviv Sourasky Medical Center and Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel H. Munoz, University of Chile Hospital, Fetal Medicine Unit, Obstetrics & Gynecology, Santiago, Chile D. Paladini, Fetal Medicine and Surgery Unit, Istituto G. Gaslini, Genoa, Italy F. Prefumo, Division of Obstetrics and Gynaecology, Department of Clinical and Experimental Sciences, University of Brescia, Brescia, Italy A. Sotiriadis, Second Department of Obstetrics and Gynecology, School of Medicine, Aristotle University of Thessaloniki, Thessaloniki, Greece A. Toi, Medical Imaging, Mount Sinai Hospital, University of Toronto, Toronto, ON, Canada W. Lee, Baylor College of Medicine, Department of Obstetrics and Gynecology, Houston, TX, USA These guidelines were developed by the Prenatal Ultrasound Screening Task Force under the auspices of the ISUOG Clinical Standards Committee. Appreciation is extended, in particular, to specialty consultants who contributed to this project, notably Jacques Abramowicz (USA) for his contribution to the Safety section, and Jean-Philippe Bault (France) for providing some of the images. These Guidelines should be cited as: ‘Salomon LJ, Alfirevic Z, Berghella V, Bilardo CM, Chalouhi GE, Da Silva Costa F, Hernandez-Andrade E, Malinger G, Munoz H, Paladini D, Prefumo F, Sotiriadis A, Toi A, Lee W, on behalf of the ISUOG Clinical Standards Committee. ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 2022; 59: 840–856. At least one meta-analysis, systematic review or randomized controlled trial rated as 1++ and applicable directly to the target population; or a systematic review of randomized controlled trials or a body of evidence consisting principally of studies rated as 1+ applicable directly to the target population and demonstrating overall consistency of results Body of evidence including studies rated as 2++ applicable directly to the target population and demonstrating overall consistency of results; or extrapolated evidence from studies rated as 1++ or 1+