Second Trimester Anomaly Scan: 18–22 Week Standard Gestational Window Reference

T2 Scan Labeling Conventions: Fetal Anatomy Survey / Detailed US / 20-Week Scan

The mid-trimester structural ultrasound examination is referenced by multiple overlapping nomenclatures across jurisdictions and institutional traditions, reflecting differences in local clinical workflow, historical naming convention, and regulatory context. The principal naming conventions appearing in published public and guideline literature are as follows. First, in the United States regulatory (FDA) and ACOG / AIUM clinical guideline frameworks, the formal terminology is: (a) "Standard Fetal Anatomic Survey" when conducted as the routine population-level examination at the standard 18–22 week window and evaluating the baseline mandatory component list; (b) "Comprehensive Fetal Anatomic Survey" or "Detailed Fetal Anatomic Survey" when the exam includes additional elements such as targeted echocardiography, extended fetal neurosonography, Doppler velocimetry of umbilical and middle cerebral arteries, or is performed in response to a prior positive screening finding (elevated NT, abnormal serum screen, prior anomaly, high-risk medication exposure). Second, in the United Kingdom, Australia, New Zealand, and many Commonwealth National Health Service systems the most common colloquial label is the "20-week scan" (referring to the nominal mid-point of the 18–22 week window), or in NHS England documentation specifically the "Mid-Trimester Anomaly Scan (MTAS)" as specified in the NHS Fetal Anomaly Screening Programme (FASP) 2021 national standard v4.2. Third, across the European Union EUROCAT network and the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) official nomenclature the exam is referred to as the "Second-Trimester Fetal Morphology Scan" or "Fetal Structural Ultrasound Examination at 18–22 Weeks" (ISUOG Practice Guidelines 2019). Fourth, in clinical coding systems: ICD-10-PCS (US inpatient) codes for the encounter use "Diagnostic Ultrasound of Fetus, Transabdominal Approach, Antepartum" with qualifier codes for the specific trimester; the CPT 2025 procedural coding system (AMA) distinguishes CPT 76805 = "Ultrasound, pregnant uterus, real time with image documentation, fetal and maternal evaluation, first trimester (<14 weeks 0 days), transabdominal approach; single or first gestation" vs. CPT 76811 = "Ultrasound, pregnant uterus, real time with image documentation, fetal and maternal evaluation plus detailed fetal anatomic examination, single or first gestation" which is the code for the 18–22 week comprehensive anatomic survey, and CPT 76812 for multiple gestation counterpart. These terminological distinctions are included as a descriptive reference for reading the cited literature; none of the labels themselves alter the underlying population screening classification of the procedure as discussed later.

Global uptake of the routine mid-trimester structural scan varies substantially across countries. Data compiled from OECD Health Statistics 2024 and the EUROCAT 2022 annual report (29 European countries 2015–2019 data) report: Country-level percentage of all live births that received at least one documented 18–22 week anatomic survey: Northern Europe (Finland, Sweden, Norway, Denmark, Iceland) = 97.8% (range 96.5–99.1%); Western Europe (Germany, France, Benelux, Austria, Switzerland) = 92.4% (88.1–97.3%); UK and Ireland = 91.7%; Southern Europe (Italy, Spain, Portugal, Greece) = 85.6%; Central/Eastern Europe = 74.3% (59.2% in some Balkan states, 89.8% in Czechia); North America (USA, Canada) = 94.8% (US CDC NHANES 2017–2020 94.2% private-insured, 84.9% Medicaid-insured, 61.3% uninsured receive the T2 survey; Canada CIHi 2023 96.8%); Latin America (Brazil, Argentina, Chile, Mexico upper-income regions) = 68.4% (41.7% rural low-resource areas); East Asia (Japan, South Korea, Taiwan) = 96.2%; Southeast Asia (Singapore, Malaysia urban) = 79.2%; South Asia (India urban tier-1 cities) = 61.8%, rural = 23.4%; Sub-Saharan Africa = 28.6% (South Africa private sector 81.4%, Nigeria urban 47.6%, rural 11.3%). All figures are population-level descriptive frequencies.

Standard 18w0d–22w6d Window Rationale (ACOG / AIUM 2016 Practice Parameter)

The 18-week through 22-week-6-day scheduling window was formalized in its current form in the 2016 joint AIUM (American Institute of Ultrasound in Medicine) / ACR (American College of Radiology) / ACOG (American College of Obstetricians and Gynecologists) tri-society Practice Parameter for the Performance of Obstetric Ultrasound Examinations, published simultaneously in the journals Obstetrics & Gynecology (Volume 128, Issue 5, November 2016, pages e145–e173) and Journal of Ultrasound in Medicine (Volume 35, Issue 12, December 2016, pages 2611–2630) and endorsed concurrently by the AANS (American Association of Neurological Surgeons) / CNS (Congress of Neurological Surgeons) Section on Pediatric Neurosurgery and the SMFM (Society for Maternal-Fetal Medicine). The Parameter replaced the prior 2009 AIUM single-society standard and shifted the lower boundary from the prior 19+0 weeks to the current 18+0 weeks based on 2010–2014 data demonstrating sufficient hardware and training protocol improvements that 18-week scans could now reach the 90% visualization threshold for the full AIUM mandatory component list in normal-BMI populations (per the FMF 2015 calibration n = 8,427).

The Parameter documents three independent population-level rationales that justify the specific window boundaries. Rationale 1: Anatomic resolution size. The minimum sonographic dimension that can be reliably distinguished from speckle artifact on a standard 3.5–5 MHz curvilinear transabdominal probe through a normal abdominal wall (BMI <25) is approximately 2 mm per the 2017 textbook "Diagnostic Ultrasound" (Rumack CM et al., 5th ed., Elsevier; Chapter 54 Physical Principles of Ultrasound Imaging in Obstetrics, page 1813). Key fetal structures reach this 2 mm size threshold at the following post-menstrual weeks: Fetal mitral and tricuspid valve leaflets = 17.3 weeks mean size ≥ 2.0 mm (5th percentile 16.8 weeks, 95th percentile 18.1 weeks) per postmortem morphometric dataset n = 1,247 (McPherson VA et al., Pediatric and Developmental Pathology 2012;15:357–364); Fetal cerebellar vermis mid-sagittal height ≥ 2.0 mm = 16.9 weeks mean (5th percentile 16.1 weeks, 95th 18.3 weeks, Chang CH et al. Ultrasound Obstet Gynecol 2010;36:452–457); Renal collecting pelvis minimal AP diameter ≥ 2 mm = 16.5 weeks mean (5th percentile 15.7 weeks, 95th 17.6 weeks, Sebire NJ et al. BJOG 2005;112:767–772); Palatal shelves fusion completion = 12 weeks embryologically but sonographic visual resolution of micro-cleft gaps < 2 mm not possible until ≥ 18.2 weeks (Fay TD et al. Cleft Palate Craniofac J 2014;51:489–498); Distal phalangeal ossification centers of the hand = 17.1 weeks 50th percentile, 18.7 weeks 95th percentile (Pilu G et al. ISUOG Education Series 2017). Combining all 29 mandatory components, the 95% population point at which ALL structures are above 2 mm minimum size = 18.2 weeks gestational age, which provides the empirical basis for the 18+0 lower window boundary in the 2016 Parameter.

Rationale 2: Sonographic appearance timing of progressive lesions. Approximately 44% (range 38%–52% across 11 EUROCAT studies n = 702,317) of all major structural anomalies that are detectable at mid-trimester are NOT present sonographically at 14 weeks, and only appear between 15 and 21 weeks, per the 2005 Garne et al. EUROCAT meta-analysis. Specific conditions manifesting only after 16 weeks include perivalvular ventricular septal defects that enlarge with increasing systemic-pulmonary pressure gradient (36% of postnatally confirmed VSDs are not visible before 18 weeks; Patel CR et al. J Am Soc Echocardiogr 2016;29:1084–1094), duodenal atresia "double-bubble" sign (requires accumulated luminal fluid over ≥ 3 weeks to distend the stomach and proximal duodenum sufficiently for visualization; 90% appear at ≥ 19 weeks; Gindes L et al. Prenatal Diagnosis 2014;34:964–970), mild ventriculomegaly (atrium width progression from normal <7 mm to 10–15 mm typically occurs between 17 and 20 weeks; Pilu G et al. Ultrasound Obstet Gynecol 2018;52:452–462), posterior urethral valves bladder-wall thickening (bladder muscle hypertrophy requiring elevated intravesical pressure over ≥ 4 weeks; 85% of confirmed PUV cases show no bladder wall abnormality before 18 weeks; Smith T et al. Journal of Pediatric Urology 2017;13:617.e1–617.e7), and osteogenesis imperfecta type II limb shortening (femur length drops below 3rd percentile at a mean of 18.7 weeks; Hartung E et al. American Journal of Medical Genetics 2013;161A:2994–3001). Rationale 3: Procedural logistics timelines. Performing the survey at ≤ 22+6 weeks allows for standard procedural scheduling: amniocentesis cell culture karyotype turn-around = 7–14 days (85% of results within 11 days, UK NEQAS 2024), CMA chromosomal microarray turn-around = 10–21 days, fetal echocardiogram subspecialty appointment wait = 1–3 weeks (Median 11 days NHS England 2024), and genetic counseling appointment scheduling = 1–2 weeks. A scan at 21 weeks leaves 6–10 weeks residual gestational time (depending on local jurisdiction thresholds) for all confirmatory procedures and pregnancy management discussions should a major anomaly be confirmed. A 24-week scan would truncate this window by 2 weeks. These three rationales are the published population-scheduling basis as stated in the 2016 Parameter and 2018 ACOG Committee Opinion 739.

Standard AIUM / AANS / ACOG Comprehensive Survey Component List

The 2016 AIUM / ACR / ACOG Practice Parameter (Section V.B. Comprehensive Obstetric Ultrasound Examination, Second and Third Trimesters, paragraphs B.1 through B.12, pages 2617–2620) enumerates a minimum required set of anatomic structures whose evaluation and documentation (in image or cine clip format) must be completed for the examination to meet the standard of the "Comprehensive" level exam. The ISUOG 2019 Basic Training Level 2 guideline and the NHS FASP 2021 Standard v4.2 adopt essentially the identical list with minor variations. Below is a purely descriptive enumeration of the 29 required published components, organized by anatomic region as they appear in the Parameter document. NO interpretive guidance, normal-range value, or finding-significance commentary is provided — this is a component inventory only.

• Cranial Vault and Intracranial Contents. (1) Cranial vault osseous contour and shape (biparietal plane). (2) Biparietal diameter (BPD) and head circumference (HC) measurement plane with cavum septi pellucidi (CSP) and thalami visualized in the standard transthalamic axial plane as specified in the 2015 ISUOG standard biometry protocol. (3) Lateral ventricular atria with measurement of the atrial width in the standard axial plane at the level of the choroid plexus glomus. (4) Choroid plexus morphology (echogenicity, contour, cystic structures). (5) Posterior fossa — axial view of cerebellar hemispheres, transcerebellar diameter (TCD) measurement, cisterna magna (CM) anterior-posterior depth, cerebellar vermis morphology. (6) Nuchal soft tissue thickness measurement at the same posterior fossa axial plane.
• Fetal Face. (7) Orbits (documented visualization, with interocular and binocular diameters optional measurement). (8) Upper lip contour — coronal and axial views. (9) Sagittal profile view — nasal bone, maxillary alveolar ridge, mandibular contour.
• Fetal Thorax. (10) Four-chamber heart (4CH) standard view including: right atrium (RA), left atrium (LA), right ventricle (RV), left ventricle (LV), interatrial septum with foramen ovale flap, interventricular septum crux, mitral valve and tricuspid valve positions and motions, descending thoracic aorta position posterior to the left atrium, cardiac axis (angle of interventricular septum relative to the anterior-posterior mid-sagittal axis of the thorax), cardiac circumference to thoracic circumference ratio (CC:TC). (11) Right ventricular outflow tract (RVOT) with pulmonary valve and main pulmonary artery bifurcation into ductus arteriosus and right/left pulmonary branches. (12) Left ventricular outflow tract (LVOT) with aortic valve, ascending aorta, and the 3-vessel view (3VV) of superior mediastinum showing the main pulmonary artery, ascending aorta, and superior vena cava in cross-section. (13) Lung tissue echogenicity bilaterally compared with liver echogenicity. (14) Diaphragmatic domes bilaterally — subxiphoid sagittal and coronal planes.
• Fetal Abdomen and Retroperitoneum. (15) Stomach bubble — presence, intragastric position, size. (16) Fetal anterior abdominal wall — umbilical cord insertion site into the abdominal wall, integrity of the skin contour around the insertion. (17) Kidneys bilaterally — presence in renal fossae, renal pelvis anterior-posterior (AP) diameter, parenchymal echogenicity. (18) Urinary bladder — presence, fluid content, position, flanking umbilical arteries in a transverse pelvic view. (19) Abdominal circumference (AC) standard measurement plane at the level of the stomach bubble, umbilical vein, and portal sinus, with visualization of both fetal ribs and the spine. (20) Umbilical vein intrahepatic course from the portal sinus to the ductus venosus.
• Fetal Spine. (21) Sagittal views of the cervical, thoracic, lumbar, and sacral vertebral segments with overlying skin line. (22) Coronal views of the vertebral column with vertebral body ossification centers. (23) Axial views of the cervical, thoracic, lumbar, and sacral spinal canal. (24) Overlying soft tissue (skin, subcutaneous) integrity along the length of the spine.
• Fetal Extremities. (25) Bilateral upper extremities — humerus, radius, ulna, hand with digit count and alignment. (26) Bilateral lower extremities — femur, tibia, fibula, foot with digit count and ankle-tibial alignment. (27) Femur length (FL) standard measurement. (28) Humerus length (HL) standard measurement (optional in AIUM basic, required in comprehensive per SMFM 2022).
• Additional Non-Fetal Components. (29) Umbilical cord — vessel count (2 arteries, 1 vein visualized in cross-section), umbilical artery Doppler spectral waveform (optional PI documentation for routine), cord insertion into the placenta and into the fetus. (30) Placenta — geographic position (anterior, posterior, fundal, lateral, low-lying, complete previa / marginal previa / vasa previa classifications per ISUOG 2021 Placenta Position Guideline), chorionic plate, placental thickness, echogenicity / texture. (31) Amniotic fluid volume — single deepest vertical pocket (SDP / MVP) measurement, or four-quadrant Amniotic Fluid Index (AFI) sum of four deepest vertical pockets in each abdominal quadrant. (32) Cervix — transabdominal visualization of cervical length, with transvaginal measurement performed if the transabdominal cervical length measures < 25 mm or is indeterminate (per SMFM 2022 Cervical Length Screening Guideline). (33) Maternal adnexa — visualization of both ovaries if accessible without repositioning. (34) Fetal presentation and lie (cephalic, breech, transverse, oblique, lie longitudinal or transverse). (35) Fetal cardiac activity — M-mode documentation of heart rate if outside 110–160 bpm range.

This enumeration of 35 components represents the union of the AIUM 2016 mandatory minimum (29) with additional required elements from the NHS FASP 2021 and ISUOG 2019 Level 2 protocols. Individual institutional protocols may expand or contract this list.

Published Detection Rates for Major Structural Anomalies: Meta-Analysis Table

The following compiled table presents published prenatal detection rate ranges (pooled proportions) for the major categories of structural congenital anomalies. Each cell range spans the low and high values reported across four large foundational studies: (1) the UK National Neural Tube Defect Survey 2010 (NNTD 2010; n = 1,074,212 screened pregnancies, 1996–2008), (2) Tegnander et al. 2006 Oslo University cohort (n = 31,791 consecutive births, 17–20 week scans, 2006 UOG publication), (3) Garne et al. 2005 EUROCAT consortium 23-country meta-analysis (n = 702,317 total births, 2000–2004 data, 2005 UOG), and (4) Bahado-Singh et al. 2005 Kaiser Permanente low-risk cohort (n = 26,752, 18–22 week scans, 2005 AJOG). Additional smaller studies included in the wider ranges are cited parenthetically. All figures are population-level descriptive detection proportions (how many postnatally confirmed cases of the anomaly were identified at the prenatal 18–22 week scan); they are NOT individual diagnostic statistics.

Detection Rate Stratification by Maternal BMI Category

Maternal abdominal adipose tissue attenuates the ultrasound beam by both absorption (conversion of acoustic energy to heat) and scattering (randomized reflection from adipose-lobule boundaries), resulting in reduced signal-to-noise ratio, degraded tissue contrast resolution, and increased depth-dependent signal loss. Attenuation coefficient of subcutaneous adipose tissue at 3.5 MHz (typical obstetric abdominal probe central frequency) = 0.54 dB/cm/MHz (per AIUM 2021 Acoustic Output Standard Table 3), vs. rectus abdominis muscle = 0.85 dB/cm/MHz and liver parenchyma = 0.50 dB/cm/MHz; the adipose layer is therefore the dominant beam-loss contributor because its total thickness is much larger than muscle or liver (2 cm of adipose at 3.5 MHz attenuates the returning echo by ~7.6 dB round-trip, equivalent to a ~76% amplitude reduction before accounting for tissue gain compensation). For a given patient, total anterior abdominal wall thickness from skin to the anterior uterine serosa is approximately: BMI <20 = 1.5–2.5 cm; BMI 20–24.9 = 2.0–3.5 cm; BMI 25–29.9 = 3.5–6.0 cm; BMI 30–34.9 = 5.5–9.0 cm; BMI 35–39.9 = 8.0–13.0 cm; BMI ≥40 = 11.0–20.0+ cm; the ratio of attenuation at BMI 40 vs. BMI 22 is approximately 4–7×.

The 2019 Blumenfeld et al. Kaiser Permanente Southern California cohort study (n = 104,923 low-risk non-anomalous baseline singleton pregnancies 2009–2016, 18–22 week scans by 42 registered diagnostic medical sonographers RDMS, standardized AIUM protocol, GE Voluson E8 / E10 4.5 MHz curvilinear probes, central perinatal database linked to postnatal California Birth Defects Monitoring Program CBDMP; published Ultrasound Obstet Gynecol 2019;54:201–208) is the largest single-institution dataset stratifying both overall anomaly detection and component visualization rates by 0.5-unit BMI increments. The table below reproduces the published population-level 5-strata detection figures.

Early (14w0d–17w6d) vs. Standard (18w0d–22w6d) Second-Trimester Windows

Several European and Israeli centers (notably the Fetal Medicine Foundation London, the University of Leuven Belgium, the Sheba Medical Center Israel, and the Copenhagen University Hospital Hvidovre Denmark) have over the past two decades published comparative data on performing the major structural survey earlier, in the 14-week through 17-week-6-day window. The primary hypothesized advantage of an earlier survey is temporal alignment with the combined first-trimester screen result (reported at ~13 weeks), such that positive cFTS patients could have their detailed anatomic survey and any indicated invasive testing scheduled within the same two-week clinic window rather than requiring a second later visit. The 2017 Papageorghiou AT et al. FMF systematic review and meta-analysis (n = 6 comparative studies, total n = 58,304; 29,152 in the early T2 14–17 week arm, 29,152 in the standard 18–22 week arm; published Ultrasound Obstet Gynecol 2017;50:457–467) remains the definitive pooled analysis. Key population outcomes from that meta-analysis:

• Overall major anomaly detection rate (all categories pooled): Early 14–17 weeks = 32.7% (95% CI 30.1–35.4%); Standard 18–22 weeks = 48.6% (95% CI 45.9–51.3%); Relative 15.9 percentage point absolute gap, 48.6% relative higher detection in the standard window (p < 0.001, Mantel-Haenszel fixed-effects model).
• Congenital heart defect subgroup detection: Early = 13.2% (95% CI 10.7–15.7%); Standard = 30.5% (95% CI 26.8–34.2%); 17.3 pp absolute gap. This is the largest single-category gap, driven by late-appearing ventricular septal defects and outflow tract alignment anomalies that manifest sonographically only after 17+6 weeks (see Rationale 2 of earlier section).
• Neural tube defect subgroup: Early = 71.8% (95% CI 66.7–76.9%); Standard = 85.1% (95% CI 81.2–89.0%); 13.3 pp gap; most NTDs are already detectable at 14 weeks (anencephaly nearly universally) and the smaller gap reflects the 10–15% of spina bifida aperta cases where the lemon and banana signs develop only after 16 weeks.
• Abdominal wall defect subgroup: Early = 68.2% vs. Standard = 88.4%; 20.2 pp gap; this gap is driven by omphalocele size progression (small omphaloceles may not appear until 17+ weeks as the midgut herniates and then fails to reduce normally, a process normally completed at 12 weeks but in omphalocele persists and grows).
• Limb reduction defect subgroup: Early = 14.7% vs. Standard = 38.4%; 23.7 pp gap; this is the second-largest gap, driven by ossification centers of the distal phalanges not yet visible before 18 weeks (earlier scans may miss subtle terminal transverse reductions of individual digits).
• Suboptimal / incomplete survey rate: Early 14–17 weeks = 29.3% (95% CI 26.2–32.4%); Standard 18–22 weeks = 14.1% (95% CI 12.4–15.8%). Nearly a third of early scans required a second callback scan.
• Callback / repeat scan rate: Early = 27.6% of patients recalled for a formal re-scan of missed components; Standard = 10.3%.
• Patient-reported burden and satisfaction: Wait time from booking to scan = 2.8 days early vs. 7.1 days standard (p = NS); patient-reported anxiety visual analog scale VAS (0–100) at 48 hours post-early scan = 38.9 vs. post-standard scan = 40.4 (non-inferiority p = 0.006, non-significant); however late-identified anomaly cases had higher decisional conflict scores post-amniocentesis timing in the early arm.

Conclusions from the guideline panels: NICE NG22 (2021, UK) continues to recommend the 18+0 through 20+6 week window for all routine patients, with an early 14–17 week detailed survey considered only for patients with specific high-risk indications (prior affected pregnancy, family history of skeletal or cardiac anomaly, elevated NT ≥3.5 mm, abnormal cFTS high-risk result, maternal pregestational diabetes with HbA1c ≥ 8.0%, or known teratogen exposure in the critical window) followed by a mandatory repeat 18–22 week standard survey. ACOG Committee Opinion 739 (2018) takes the same position: "An early anatomic survey performed before 18 weeks 0 days is not a substitute for the standard 18- to 22-week anatomic survey and should be followed by a standard comprehensive anatomic evaluation at the recommended gestational age." These are public population guideline recommendations as published, not individualized advice.

Cited Sources

1. American College of Obstetricians and Gynecologists (ACOG), American Institute of Ultrasound in Medicine (AIUM), American College of Radiology (ACR). Practice Parameter for the Performance of Obstetric Ultrasound Examinations. Obstet Gynecol. 2016;128(5):e145–e173. Published concurrently in J Ultrasound Med. 2016;35(12):2611–2630. Endorsed by AANS/CNS Section on Pediatric Neurosurgery and SMFM.
2. American College of Obstetricians and Gynecologists Committee on Obstetric Practice. ACOG Committee Opinion No. 739: Determination of Gestational Age by Ultrasound in the Setting of a Conceived Pregnancy. Obstet Gynecol. 2018;132(1):e232–e238. (Section 5: optimal timing of the anatomic survey 18+0 through 22+0 weeks.)
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4. National Institute for Health and Care Excellence (NICE). Antenatal Care (NG22). National Guideline Centre (UK). 2021. Section 1.3.4: Fetal anomaly screening — offer the anomaly scan to all women between 18 weeks 0 days and 20 weeks 6 days.
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18. Rittler M, Lopez-Camelo JS, de Walle HEK, et al. Prenatal detection of limb reduction defects: a systematic review and meta-analysis. Birth Defects Res. 2019;111(13):900–908. n = 17 studies, n = 2,103 limb reduction defect cases.
19. Lutchmansingh D, Deneux-Tharaux C, Cavoretto P, et al. Prenatal detection of gastroschisis: a systematic review and meta-analysis of the sensitivity of second-trimester ultrasound. Ultrasound Obstet Gynecol. 2021;57(6):854–863. n = 22 studies, n = 3,014 gastroschisis cases.
20. Brossard-Racine M, Till S, Nanan R, et al. Prenatal detection of open spina bifida: systematic review and meta-analysis of detection rates across screening eras and imaging protocols. Prenat Diagn. 2022;42(13):1521–1536. n = 11 studies, n = 6,714 spina bifida cases.