<scp>ISUOG</scp> Practice Guidelines: performance of first‐trimester fetal ultrasound scan

Ultrasound in Obstetrics & GynecologyVolume 41, Issue 1 p. 102-113 ISUOG GuidelinesFree Access ISUOG Practice Guidelines: performance of first-trimester fetal ultrasound scan First published: 25 December 2012 https://doi.org/10.1002/uog.12342Citations: 341 (Guideline review date: June 2015) The authorship of this article was incomplete as initially published. This version of the article correctly acknowledges all authors who contributed to the development of the Guidelines. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Clinical Standards Committee The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) is a scientific organization that encourages safe 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 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 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 accept any 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). Introduction Routine ultrasound examination is an established part of antenatal care if resources are available and access possible. It is commonly performed in the second trimester1, although routine scanning is offered increasingly during the first trimester, particularly in high-resource settings. Ongoing technological advancements, including high-frequency transvaginal scanning, have allowed the resolution of ultrasound imaging in the first trimester to evolve to a level at which early fetal development can be assessed and monitored in detail. The aim of this document is to provide guidance for healthcare practitioners performing, or planning to perform, routine or indicated first-trimester fetal ultrasound scans. ‘First trimester’ here refers to a stage of pregnancy starting from the time at which viability can be confirmed (i.e. presence of a gestational sac in the uterine cavity with an embryo demonstrating cardiac activity) up to 13 + 6 weeks of gestation. Ultrasound scans performed after this gestational age are not considered in these Guidelines. Throughout these Guidelines we use the term ‘embryo’ for before 10 weeks and ‘fetus’ thereafter, to reflect the fact that after 10 weeks of gestation organogenesis is essentially complete and further development involves predominantly fetal growth and organ maturation2, 3. General Considerations What is the purpose of a first-trimester fetal ultrasound scan? In general, the main goal of a fetal ultrasound scan is to provide accurate information which will facilitate the delivery of optimized antenatal care with the best possible outcomes for mother and fetus. In early pregnancy, it is important to confirm viability, establish gestational age accurately, determine the number of fetuses and, in the presence of a multiple pregnancy, assess chorionicity and amnionicity. Towards the end of the first trimester, the scan also offers an opportunity to detect gross fetal abnormalities and, in health systems that offer first-trimester aneuploidy screening, measure the nuchal translucency thickness (NT). It is acknowledged, however, that many gross malformations may develop later in pregnancy or may not be detected even with appropriate equipment and in the most experienced of hands. When should a first-trimester fetal ultrasound scan be performed? There is no reason to offer routine ultrasound simply to confirm an ongoing early pregnancy in the absence of any clinical concerns, pathological symptoms or specific indications. It is advisable to offer the first ultrasound scan when gestational age is thought to be between 11 and 13 + 6 weeks' gestation, as this provides an opportunity to achieve the aims outlined above, i.e. confirm viability, establish gestational age accurately, determine the number of viable fetuses and, if requested, evaluate fetal gross anatomy and risk of aneuploidy4-20. Before starting the examination, a healthcare provider should counsel the woman/couple regarding the potential benefits and limitations of the first-trimester ultrasound scan. (GOOD PRACTICE POINT) Who should perform the first-trimester fetal ultrasound scan? Individuals who perform obstetric scans routinely should have specialized training that is appropriate to the practice of diagnostic ultrasound for pregnant women. (GOOD PRACTICE POINT) To achieve optimal results from routine ultrasound examinations it is suggested that scans should be performed by individuals who fulfill the following criteria: have completed training in the use of diagnostic ultrasonography and related safety issues; participate in continuing medical education activities; have established appropriate care pathways for suspicious or abnormal findings; participate in established quality assurance programs21. What ultrasonographic equipment should be used? It is recommended to use equipment with at least the following capabilities: real-time, gray-scale, two-dimensional (2D) ultrasound; transabdominal and transvaginal ultrasound transducers; adjustable acoustic power output controls with output display standards; freeze frame and zoom capabilities; electronic calipers; capacity to print/store images; regular maintenance and servicing. How should the scan be documented? An examination report should be produced as an electronic and/or paper document (see Appendix for an example). Such a document should be stored locally and, in accordance with local protocol, made available to the woman and referring healthcare provider. (GOOD PRACTICE POINT) Is prenatal ultrasonography safe during the first trimester? Fetal exposure times should be minimized, using the shortest scan times and lowest possible power output needed to obtain diagnostic information using the ALARA (As Low As Reasonably Achievable) principle. (GOOD PRACTICE POINT) Many international professional bodies, including ISUOG, have reached a consensus that the use of B-mode and M-mode prenatal ultrasonography, due to its limited acoustic output, appears to be safe for all stages of pregnancy22, 23. Doppler ultrasound is, however, associated with greater energy output and therefore more potential bioeffects, especially when applied to a small region of interest24, 25. Doppler examinations should only be used in the first trimester, therefore, if clinically indicated. More details are available in the ISUOG Safety Statement22. What if the examination cannot be performed in accordance with these Guidelines? These Guidelines represent an international benchmark for the first-trimester fetal ultrasound scan, but consideration must be given to local circumstances and medical practices. If the examination cannot be completed in accordance with these Guidelines, it is advisable to document the reasons for this. In most circumstances, it will be appropriate to repeat the scan, or to refer to another healthcare practitioner. This should be done as soon as possible, to minimize unnecessary patient anxiety and any associated delay in achieving the desired goals of the initial examination. (GOOD PRACTICE POINT) What should be done in case of multiple pregnancies? Determination of chorionicity and amnionicity is important for care, testing and management of multifetal pregnancies. Chorionicity should be determined in early pregnancy, when characterization is most reliable26-28. Once this is accomplished, further antenatal care, including the timing and frequency of ultrasound examinations, should be planned according to the available health resources and local guidelines. (GOOD PRACTICE POINT) Guidelines For Examination 1. Assessment of viability/early pregnancy In this Guideline, ‘age’ is expressed as menstrual or gestational age, which is 14 days more than conceptional age. Embryonic development visualized by ultrasound closely agrees with the ‘developmental time schedule’ of human embryos described in the Carnegie staging system3. The embryo is typically around 1–2 mm long when first detectable by ultrasound and increases in length by approximately 1 mm per day. The cephalic and caudal ends are indistinguishable until 53 days (around 12 mm), when the diamond-shaped rhombencephal cavity (future fourth ventricle) becomes visible18. Defining viability The term ‘viability’ implies the ability to live independently outside the uterus and, strictly speaking, cannot be applied to embryonic and early fetal life. However, this term has been accepted in ultrasound jargon to mean that the embryonic or fetal heart is seen to be active and this is taken to mean the conceptus is ‘alive’. Fetal viability, from an ultrasound perspective, is therefore the term used to confirm the presence of an embryo with cardiac activity at the time of examination. Embryonic cardiac activity has been documented in normal pregnancies at as early as 37 days of gestation29, which is when the embryonic heart tube starts to beat30. Cardiac activity is often evident when the embryo measures 2 mm or more31, but is not evident in around 5–10% of viable embryos measuring between 2 and 4 mm32, 33. Defining an intrauterine pregnancy The presence of an intrauterine gestational sac clearly signifies that the pregnancy is intrauterine, but the criteria for the definition of a gestational sac are unclear. The use of terms such as an ‘apparently empty’ sac, the ‘double-decidual ring’ or even ‘pseudosac’ do not accurately confirm or refute the presence of an intrauterine pregnancy. Ultimately, the decision is a subjective one and is, therefore, influenced by the experience of the person performing the ultrasound examination. In an asymptomatic patient, it is advisable to wait until the embryo becomes visible within the intrauterine sac as this confirms that the ‘sac’ is indeed a gestational sac. (GOOD PRACTICE POINT) 2. Early pregnancy measurements The mean gestational sac diameter (MSD) has been described in the first trimester from 35 days from the last menstrual period onwards. The MSD is the average of the three orthogonal measurements of the fluid-filled space within the gestational sac34. Nomograms for both crown–rump length (CRL) and MSD are available, but, in the presence of the embryo, the CRL provides a more accurate estimation of gestational age because MSD values show greater variability of age prediction35, 36. 3. First-trimester fetal measurements Which measurements should be performed in the first trimester? CRL measurements can be carried out transabdominally or transvaginally. A midline sagittal section of the whole embryo or fetus should be obtained, ideally with the embryo or fetus oriented horizontally on the screen. An image should be magnified sufficiently to fill most of the width of the ultrasound screen, so that the measurement line between crown and rump is at about 90° to the ultrasound beam37, 38. Electronic linear calipers should be used to measure the fetus in a neutral position (i.e. neither flexed nor hyperextended). The end points of crown and rump should be defined clearly. Care must be taken to avoid inclusion of structures such as the yolk sac. In order to ensure that the fetus is not flexed, amniotic fluid should be visible between the fetal chin and chest (Figure 1). However, this may be difficult to achieve at earlier gestations (around 6–9 weeks) when the embryo is typically hyperflexed. In this situation, the actual measurement represents the neck–rump length, but it is still termed the CRL. In very early gestations it is not usually possible to distinguish between the cephalic and caudal ends and a greatest length measurement is taken instead. Figure 1Open in figure viewerPowerPoint Crown–rump length (CRL) measurement technique in a fetus with CRL 60 mm (12 + 3 weeks). Note neutral position of neck. The biparietal diameter (BPD) and head circumference (HC) are measured on the largest true symmetrical axial view of the fetal head, which should not be distorted by adjacent structures or transducer pressure. At about 10 weeks' gestation, structures such as the midline third ventricle, interhemispheric fissure and choroid plexuses should be visible. Towards 13 weeks, the thalamus and third ventricle provide good landmarks. Correct axial orientation is confirmed by including in the image both anterior horns and low occipital lobes of the cerebral ventricles, whilst keeping the plane above the cerebellum1, 38-41. For BPD measurement, caliper placement should follow the technique used to produce the selected nomogram. Both outer-to-inner (leading edge) and outer-to-outer measurements are in use1, 39, 42, 43 (Figure 2). Figure 2Open in figure viewerPowerPoint Fetal head. (a) Biparietal diameter (BPD) measurement (calipers). Note true axial view through head and central position of third ventricle and midline structures (T indicates third ventricle and thalamus). Head circumference would also be measured in this plane. (b) Normal choroid plexuses (C) and midline falx and interhemispheric fissure (arrows). Note that choroid plexuses extend from the medial to the lateral border of the posterior horn. Lateral walls of anterior horns are indicated by arrowheads. Other measurements Nomograms are available for abdominal circumference (AC), femur length and most fetal organs, but there is no reason to measure these structures as part of the routine first-trimester scan. 4. Assessment of gestational age Pregnant women should be offered an early ultrasound scan between 10 + 0 and 13 + 6 weeks to establish accurate gestational age. (Grade A recommendation) Ultrasound assessment of embryonic/fetal age (dating) uses the following assumptions: gestational (menstrual age) represents post-conception age + 14 days; embryonic and fetal size correspond to post-conception (post fertilization) age; structures measured are normal; measurement technique conforms to the reference nomogram; measurements are reliable (both within and between observers); the ultrasound equipment is calibrated correctly. Accurate dating is essential for appropriate follow-up of pregnancies and has been the primary indication for routine ultrasound in the first trimester. It provides valuable information for the optimal assessment of fetal growth later in pregnancy, appropriate obstetric care in general and management of preterm or post-term pregnancies in particular44, 45. Except in pregnancies arising following assisted reproductive technology, the exact day of conception cannot be determined reliably and, therefore, dating a pregnancy by ultrasound appears to be the most reliable method with which to establish true gestational age39, 46. It has been recommended, therefore, that all pregnant women be offered an early ultrasound scan between 10 and 13 completed weeks (10 + 0 to 13 + 6 weeks) to determine gestational age and to detect multiple pregnancies47. In the first trimester, many parameters are related closely to gestational age, but CRL appears to be the most precise, allowing accurate determination of the day of conception, to within 5 days either way in 95% of cases48-52. At very early gestations, when the fetus is relatively small, measurement errors will have a more significant effect on gestational age assessment; the optimal time for assessment appears, therefore, to be somewhere between 8 and 13 + 6 weeks48. (GOOD PRACTICE POINT) At 11 to 13 + 6 weeks, the CRL and BPD are the two most commonly measured parameters for pregnancy dating. Many authors have published nomograms for these variables. Measurements can be made transabdominally or transvaginally. Singleton nomograms remain valid and can be applied in the case of multiple pregnancy27, 53. Details of a few published nomograms are provided in Table 1. It is recommended that CRL measurement should be used to determine gestational age unless it is above 84 mm; after this stage, HC can be used, as it becomes slightly more precise than is BPD41. (GOOD PRACTICE POINT) Table 1. Biometry nomograms for consideration in first trimester to around 13 + 6 weeks Reference Structure measured Age range (weeks) Notes Robinson & Fleming52 (1975); quoted by Loughna et al.41 (2009) CRL 9 to 13 + 6 Selected for use by British Medical Ultrasound Society41 Hadlock et al.83 (1992) CRL 5.0 to 18.0 Daya84 (1993) CRL 6.1 to 13.3 Verburg et al.43 (2008) CRL 6 + 2 to 15 + 0 Includes BPD, HC, AC, femur, cerebellum McLennan & Schluter85 (2008) CRL 5 to 14 Includes BPD to 14 weeks Hadlock et al.86 (1982) BPD 12 to 40 In early pregnancy 1982 chart more accurate than 1984 chart Altman & Chitty39 (1997); quoted by Loughna et al.41 (2009) BPD 12 + 6 to 35 + 4 Selected for use by British Medical Ultrasound Society41 Verburg et al.43 (2008) BPD 10 to 43 Includes CRL, HC, AC, femur, cerebellum Measurements should be performed according to techniques described in these articles and tested on the local population before being adopted into practice. BPD, biparietal diameter; CRL, crown–rump length. 5. Assessment of fetal anatomy The second-trimester ‘18–22-week’ scan remains the standard of care for fetal anatomical evaluation in both low-risk and high-risk pregnancies54-57. First-trimester evaluation of fetal anatomy and detection of anomalies was introduced in the late 1980s and early 1990s with the advent of effective transvaginal probes58, 59. The introduction of NT aneuploidy screening in the 11 to 13 + 6-week window has rekindled an interest in early anatomy scanning (Table 2). Reported advantages include early detection and exclusion of many major anomalies, early reassurance to at-risk mothers, earlier genetic diagnosis and easier pregnancy termination if appropriate. Limitations include need for trained and experienced personnel, uncertain cost/benefit ratio and late development of some anatomical structures and pathologies (e.g. corpus callosum, hypoplastic left heart), which make early detection impossible and can lead to difficulties in counseling due to the uncertain clinical significance of some findings54-56, 60-62. Table 2. Suggested anatomical assessment at time of 11 to 13 + 6-week scan Organ/anatomical area Present and/or normal ? Head Present Cranial bones Midline falx Choroid-plexus-filled ventricles Neck Normal appearance Nuchal translucency thickness (if accepted after informed consent and trained/certified operator available)aa from et et et and et with from et et et and et from et et et and et Normal from et et et and et from et et et and et and from et et et and et from et et et and et or Cardiac regular activity symmetrical from et et et and et in left from et et et and et from et et et and et Normal with three and with normal from et et et and et and from et et et and et a from et et et and et Head Cranial should be visible by 11 completed weeks (Figure It is to for in the axial and or of the should be The cerebral region at 11 to 13 + 6 weeks is by lateral ventricles that and are with the choroid plexuses in posterior two (Figure The should symmetrical and by a clearly visible interhemispheric fissure and The is very and best the fluid-filled ventricles, an appearance which should not be for At this early age, some cerebral structures (e.g. corpus callosum, are not sufficiently to accurate It has been that the posterior translucency can be between 11 and 13 + 6 weeks as a screening for tube but this is not a At 11 to 13 + 6 weeks, an can be made to the with the including the the and as as the of the and (Figure However, in absence of to the fetal at this time should not further examination earlier than the scan. Figure in figure viewerPowerPoint Fetal (a) Normal bones Note normal length of and (b) Normal with and visible. Fetal at 13 Note and line between the is Neck assessment of NT is part of the screening for anomalies and is should be to of the with the and of fluid such as and and axial should be to show normal and and an should be made to show (Figure However, in the absence of to the at this time should not further examination earlier than the scan. should be to the normal appearance of the when BPD Figure in figure viewerPowerPoint Fetal is visible posterior to the from to in a true Note show but which are still are or In region the have not and the is this is The chest of on evidence of or or should be normal position of and The normal position of the heart on the left of the chest should be documented (Figure More assessment of cardiac anatomy has been to be at 11 to 13 + 6 but this is not part of routine For safety use of Doppler is not indicated during routine Figure in figure viewerPowerPoint section of the fetal at the level of the view of the with the cardiac to the left Note and ventricles are symmetrical on either of the are of and is to left of At 11 to 13 + 6 weeks, the and are the only fluid structures in the (Figure and The position of the on the left of the with confirm normal The fetal should be in as slightly structures with central (Figure 12 weeks of gestation, the fetal should be visible as a in the Figure in figure viewerPowerPoint Fetal (a) view of at level at which abdominal circumference is measured and (b) view of with central and Note that the two are visible. The normal of the should be documented after 12 weeks (Figure The is up to 11 weeks and should be from and The presence of of the and and presence and normal orientation of the two and should be at the 11 to 13 + 6-week ultrasound scan. The of the may be visible at 11 weeks, especially with transvaginal scanning (Figure Figure in figure viewerPowerPoint Fetal (a) Normal normal of and (b) Normal normal orientation of with to visible are and The evaluation of and is the orientation of the in the but is not sufficiently accurate to be used for clinical The number of at the and presence of should be evaluation of the region with or power Doppler can be in the presence of two but this is not part of the routine of and ultrasound and ultrasound are not used for routine first-trimester fetal anatomical as resolution is not as good as that of In these may be in evaluation of especially of assessment screening for anomalies in the first trimester may be on health trained and of healthcare resources. The first-trimester screening should include NT performance is further by the of including measurement of or human and In appropriate circumstances, aneuploidy including and may be by with appropriate training and that NT should be measured between 11 and 13 + 6 weeks, to a CRL measurement of between and 84 This gestational age window is because NT as a screening and fetal size diagnosis of major fetal women who are an fetus with the of an early termination of NT to be in including counseling and management as as with specialized training and continuing details can be from and such as The Fetal However, even in the absence of screening evaluation of the nuchal region of any fetus is recommended and, if it appears should be