🧬 Fetal Anomaly Ultrasound Anatomy Scan Simulator

Head Biometry & Intracranial Survey

The detailed (Level I) fetal anatomy scan is optimally performed at 18–22 weeks — late enough for reliable visualization of small structures like the four cardiac chambers and cerebellum, yet early enough that if a major anomaly is found there is still time for confirmatory testing, subspecialist counseling, and informed decision-making. The survey begins, by convention, at the head.

• 18–22 wk: Optimal scan window (ISUOG/AIUM standard timing)
• <10 mm: Lateral ventricle atrium (upper limit of normal width)
• 1–2%: Choroid plexus cyst rate (of normal 18–22 wk scans)
• 85–90%: Major CNS anomaly detection (sensitivity, expert centers)

Purpose and timing of the detailed anatomy scan

The mid-trimester anatomy scan (often called the "anomaly scan" or Level I ultrasound) has one central purpose: systematically image every major fetal organ system against a published checklist so that structural anomalies are identified while gestational age still allows meaningful counseling, further testing, or in-utero/perinatal planning.

18–22 weeks is the sweet spot for this exam. Before ~16 weeks many structures (four-chamber heart, cerebellum, kidneys, stomach, bladder) are too small to reliably assess; ossification of the skull and spine is incomplete. After ~24 weeks, fetal position, decreasing relative amniotic fluid, and shadowing from ossified bone increasingly obscure key views, and the window for some diagnostic or management decisions narrows.

Critically, a normal anatomy scan does not exclude all anomalies. Detection rates vary enormously by organ system, anomaly severity, maternal body habitus, fetal position, equipment, and operator experience — anywhere from under 40% (some cardiac lesions on 4-chamber view alone) to over 90% (anencephaly, gastroschisis). Patients should be counseled that this is a screening exam with meaningful but incomplete sensitivity, not a guarantee of a structurally normal baby.

ISUOG and AIUM practice guidelines both specify a minimum standard content list for the mid-trimester scan; deviation (e.g., skipping outflow tracts or hands/feet) measurably lowers anomaly detection rates in population studies.

Standard head and brain planes

Three transverse planes anchor the neuro-survey:

• Transventricular plane: measures the lateral ventricle atrium (posterior horn) at the glomus of choroid plexus — normal <10 mm; ≥10 mm defines ventriculomegaly and prompts a detailed neuro-survey and possible fetal MRI. • Transthalamic plane: the workhorse biometry plane — biparietal diameter (BPD) and head circumference (HC) are measured here, at the level of the thalami and cavum septum pellucidum, using outer-to-outer/outer-to-inner caliper convention. • Transcerebellar plane: angled posteriorly to measure cerebellar diameter and the cisterna magna (normal 2–10 mm) — key for detecting Dandy-Walker malformation, cerebellar hypoplasia, or (with the "banana sign") open spina bifida.

HC and BPD, combined with femur length and abdominal circumference, feed standard growth formulas used throughout the remainder of pregnancy for fetal weight estimation.

Choroid plexus cysts as a soft marker

Choroid plexus cysts (CPCs) are fluid-filled spaces within the choroid plexus, seen in roughly 1–2% of pregnancies at the anatomy scan. In isolation, with normal serum/cfDNA screening and no other structural findings, an isolated CPC is a normal variant — over 90% resolve spontaneously by 26–28 weeks and carry no independent increase in adverse outcome.

Historically CPCs were linked to trisomy 18 (Edwards syndrome); the modern approach is risk-based: an isolated CPC in a patient with a low-risk cfDNA or combined first-trimester screen requires no further testing, while a CPC accompanied by other soft or major markers, or in a patient with an elevated a priori risk, warrants discussion of cfDNA screening or diagnostic testing.

Face, Lips, Palate Screening & Spinal Integrity

Moving down from the calvarium, the sonographer captures the fetal profile and a palate screening view, then fans the transducer along the entire vertebral column. Facial clefts and open neural tube defects are among the anomalies most sensitive to a systematic, ordered survey technique.

• ~1/700: Cleft lip ± palate (live births, most common facial cleft)
• <30%: Isolated cleft palate detection (2D ultrasound sensitivity)
• ~1/1,000–2,000: Open spina bifida prevalence (varies by folate fortification)
• ~70%: Folic acid risk reduction (periconceptional 400 mcg/day)

Profile, lips and the retronasal triangle

The mid-sagittal profile view assesses forehead contour, nasal bone presence/length, and mandible size (micrognathia can be a soft marker for trisomy 18 or a sign of a skeletal/genetic syndrome). The coronal "lips view" screens for cleft lip, seen as a discontinuity in the upper lip contour.

Cleft lip (with or without cleft palate) is detected reasonably well by 2D ultrasound — sensitivity 75–90% at expert centers — because the lip contour is directly visualized. Isolated cleft palate is far harder: without lip involvement there is no easily visualized contour break, and 2D detection sensitivity is often quoted below 30%. The retronasal triangle (a coronal view showing the two frontal maxillary processes and the palate as a triangle) and 3D ultrasound improve palate visualization but do not fully close this detection gap — a normal-appearing lips view does not exclude an isolated cleft palate.

Spinal integrity — sagittal, axial and coronal sweep

The spine is surveyed in three planes along its full length:

• Sagittal: confirms a smooth, continuously tapering vertebral column with intact overlying skin — the classic screen for a sacral/lumbar defect or soft-tissue mass. • Axial: at each vertebral level the three ossification centers should form a closed triangle/circle; splaying ("open" posterior elements) suggests spina bifida. • Coronal: two parallel ossification columns should converge normally at the sacrum without widening.

Cranial "soft markers" indirectly flag open spina bifida even when the spine itself is hard to see: the "lemon sign" (scalloped frontal bones) and "banana sign" (abnormally curved cerebellum with an effaced cisterna magna) result from Chiari II-related downward traction on the hindbrain, and together are highly sensitive indirect signs prompting a focused spinal re-scan.

Periconceptional folic acid (400 mcg/day, started ≥1 month before conception) reduces open neural tube defect risk by roughly 70%, and remains the single most effective primary-prevention measure available for this anomaly class.

Why the survey follows a fixed head-to-toe order

ISUOG and AIUM practice guidelines specify a standard checklist content and, implicitly, a systematic scanning order (head → face/spine → thorax/heart → abdomen → limbs → placenta/cord/fluid). This is not merely tidy documentation practice — audits repeatedly show that ad hoc, unsystematic scanning technique measurably increases missed-structure rates.

A fixed order: • Builds a checklist habit that resists interruption (a phone call, a difficult fetal position) without losing track of what has and has not been surveyed • Groups anatomically and embryologically related structures together (e.g., cranial soft markers reviewed alongside spine), improving pattern recognition • Creates a reproducible structure for documentation and medico-legal defensibility, and allows the whole team (sonographer, reviewing physician) to audit completeness at a glance

Heart — Four-Chamber View, Outflow Tracts & Situs

Congenital heart disease (CHD) is the single most common category of major structural birth defect, yet it remains one of the hardest anomalies to detect on ultrasound — small chamber size, constant motion, and the need for several precise planes beyond the four-chamber view all work against the sonographer.

• ~8/1,000: CHD birth prevalence (live births, all severities)
• ~40–60%: 4-chamber view alone, detection (misses many outflow lesions)
• ~70–90%: 4-chamber + outflow tracts, detection (expert center estimate)
• 3–5%: Echogenic intracardiac focus (of normal pregnancies (isolated))

The four-chamber view and cardiac situs

The axial four-chamber view is obtained just above the diaphragm and should show: two roughly equal-sized atria separated by the atrial septum (with the flap of the foramen ovale), two roughly equal-sized ventricles separated by an intact interventricular septum, the moderator band near the right ventricular apex, and the offset "cross" (crux cordis) where the mitral and tricuspid valve insertions meet — offset tricuspid septal insertion is normal and its absence suggests an atrioventricular septal defect.

Situs is confirmed by checking that the stomach bubble, cardiac apex, and (on situs-appropriate imaging) the abdominal aorta/IVC relationship are all on the correct (left-predominant) side — situs inversus or ambiguus (heterotaxy) is itself associated with complex CHD and warrants dedicated fetal echocardiography.

Why the four-chamber view alone is not enough

A structurally normal four-chamber view can coexist with serious CHD — transposition of the great arteries, tetralogy of Fallot, and many outflow tract lesions can look entirely normal at the four-chamber level because the septal and chamber anatomy is unaffected; the abnormality lies in how the great vessels arise and cross.

Current guidelines therefore extend the basic cardiac screen to include the left ventricular outflow tract (aorta arising from the LV) and right ventricular outflow tract (pulmonary artery arising from the RV, crossing the aorta) — the "extended cardiac screen." Adding these two outflow views roughly doubles detection sensitivity for major CHD compared with the four-chamber view in isolation, which is why they are now standard rather than optional in ISUOG/AIUM protocols.

Any suspected or confirmed structural cardiac finding — or a four-chamber/outflow view that cannot be adequately obtained — is an indication for referral to fetal echocardiography, a dedicated, higher-resolution cardiac-only ultrasound performed by a pediatric/fetal cardiologist.

Echogenic intracardiac focus as a soft marker

An echogenic intracardiac focus (EIF) — a small bright spot, usually in the left ventricle, representing focal mineralization of a papillary muscle — is seen in roughly 3–5% of normal pregnancies (higher in some Asian populations) and is not itself a structural cardiac defect; it does not affect cardiac function.

Isolated EIF with normal aneuploidy screening requires no further testing. It historically carried a modest association with trisomy 21, so in a patient without prior screening, or with other soft markers present, it is one input into an overall risk recalculation rather than a stand-alone indication for invasive testing.

Abdomen — Stomach, Kidneys, Bladder, Cord & Wall

The abdominal survey is dense with checklist items in a small anatomic space: the stomach bubble confirms swallowing and situs, both kidneys and the bladder confirm the urinary tract, and the cord insertion site confirms an intact anterior abdominal wall — a region responsible for several of the most common soft markers and some of the most visually dramatic major anomalies.

• 0.5–1.8%: Echogenic bowel prevalence (of mid-trimester scans)
• ≥4 mm: Mild pyelectasis threshold (AP renal pelvis, 16–20 wk)
• ~1/2,000–4,000: Gastroschisis prevalence (births, rising in young mothers)
• ~1/4,000–6,000: Omphalocele prevalence (births, ~50% with other anomalies)

Core abdominal checklist

The abdominal circumference plane (used for growth biometry) also demonstrates the stomach bubble on the fetal left — its absence, or a persistently small/non-visualized stomach, raises concern for esophageal atresia or a swallowing/neuromuscular problem, especially with polyhydramnios.

Both kidneys are identified adjacent to the spine, and the anteroposterior renal pelvis diameter is measured; the bladder is confirmed as a midline anechoic structure with two umbilical arteries coursing alongside it on color Doppler. The cord insertion site is scanned specifically to confirm an intact abdominal wall with the cord entering as a normal three-vessel structure with no herniated bowel or liver.

Echogenic bowel and mild pyelectasis as soft markers

Echogenic bowel — fetal bowel appearing as bright as adjacent bone on ultrasound — is seen in roughly 0.5–1.8% of mid-trimester scans. It is a nonspecific finding with several possible associations: swallowed blood, cystic fibrosis (fetus is a CF carrier or affected), congenital CMV or toxoplasmosis infection, fetal growth restriction, and a modestly increased aneuploidy risk (particularly T21). Work-up typically includes CF carrier screening of both parents, TORCH serology, aneuploidy risk reassessment, and growth surveillance; most cases with a negative work-up have a good outcome.

Mild pyelectasis (renal pelvis dilation ≥4 mm at 16–20 weeks, or ≥7 mm at 20–30 weeks) is common and most often resolves or reflects a benign, transient dilation; persistent or worsening dilation raises concern for ureteropelvic junction obstruction or vesicoureteral reflux and warrants a third-trimester follow-up scan, with postnatal renal ultrasound regardless of resolution.

Soft markers are individually common and usually benign — the clinical skill lies in synthesizing the full pattern (how many markers, which combination, background risk) rather than reacting to any single finding in isolation.

Abdominal wall defects — major anomalies

Gastroschisis is a full-thickness paraumbilical wall defect (almost always to the right of a normally inserting cord) through which bowel herniates freely into the amniotic fluid, uncovered by membrane. It is not strongly associated with aneuploidy but requires growth surveillance (bowel-related growth restriction is common) and delivery planning at a center with pediatric surgery.

Omphalocele is a midline defect at the cord insertion itself, with herniated viscera (bowel, sometimes liver) contained within a peritoneal membrane. Unlike gastroschisis, omphalocele carries a substantial (~30–50%) association with aneuploidy and other structural anomalies, and should prompt a detailed anatomic survey, genetic counseling, and diagnostic testing discussion in addition to surgical planning.

Limbs & Extremities — Long Bones, Hands, Feet

The limb survey is often rushed, yet it screens for a wide range of pathology: isolated positional foot deformities, syndromic skeletal dysplasias, and soft markers of aneuploidy risk all present here. Every long bone and every hand and foot should be visualized and, where relevant, measured.

• ~1/1,000: Clubfoot (talipes) prevalence (live births, ~50% bilateral)
• ~80%: Isolated clubfoot, normal karyotype (when no other findings present)
• <5th %ile: Short femur soft-marker threshold (for gestational age)
• ~1/2,000: Limb reduction defects (births, all types combined)

Long bone biometry and the limb checklist

All four long bones — bilateral femurs, humeri, and ideally tibia/fibula and radius/ulna — should be identified and at least the femur length measured for growth biometry and to screen for skeletal dysplasia (markedly shortened, bowed, or fractured-appearing long bones).

Both hands and both feet are visualized with an attempt to count digits (five per hand/foot) and assess hand posture (a persistently clenched fist with overlapping fingers is a soft marker seen in trisomy 18) and foot axis relative to the lower leg.

Clubfoot and other limb anomalies

Clubfoot (talipes equinovarus) is diagnosed when the foot is seen in the same imaging plane as the tibia/fibula with a fixed medial and downward deviation, rather than the normal perpendicular relationship. It occurs in roughly 1 in 1,000 births, is bilateral in about half of cases, and — when truly isolated with an otherwise normal detailed survey — is associated with a normal karyotype in the large majority (~80%) of cases, most often reflecting idiopathic or positional (in-utero crowding) etiology.

Because a minority of clubfoot cases are syndromic or associated with neuromuscular conditions (e.g., arthrogryposis) or chromosomal anomalies, current practice recommends a detailed (Level II) anatomy survey after any clubfoot diagnosis, plus counseling that isolated clubfoot is very treatable postnatally, typically with the Ponseti serial-casting method, with excellent functional outcomes.

Limb reduction defects (missing or foreshortened segments) are less common (~1/2,000 births) but are frequently associated with additional anomalies or known teratogen exposure, and always warrant a full detailed anatomic survey.

Short long bones as an aneuploidy soft marker

A femur or humerus length measuring below the 5th percentile for gestational age (or a femur/BPD ratio below normal cutoffs), in the absence of a skeletal dysplasia pattern, is a recognized soft marker modestly associated with trisomy 21. Like other isolated soft markers, an isolated short long bone with normal aneuploidy screening and no other findings is usually a benign variant of growth, but it should prompt a repeat growth assessment later in pregnancy to exclude evolving fetal growth restriction.

Placenta, Cord, Amniotic Fluid & Scan Completeness

The survey closes outside the fetus itself: placental location and appearance, umbilical cord vessel count, and amniotic fluid volume are documented, and the exam is summarized as complete or as requiring a targeted follow-up. This is also where the clinician decides whether findings warrant escalation beyond the standard scan.

• ~4–5%: Placenta previa at 20 wk (of pregnancies (most resolve))
• 5–24 cm: Normal amniotic fluid index ((or single deepest pocket 2–8 cm))
• ~1%: Single umbilical artery (of pregnancies)
• ~50–90%: Overall major anomaly detection (wide range by anomaly, BMI, center)

Placenta, cord and amniotic fluid assessment

Placental location relative to the internal cervical os is documented — placenta previa or a low-lying placenta at 20 weeks is common (~4–5%) but resolves in the large majority of cases as the lower uterine segment develops, so a follow-up third-trimester scan (rather than immediate concern) is the standard response. Placental texture, thickness, and any masses (e.g., chorioangioma) are noted.

The umbilical cord is confirmed as a three-vessel cord (two arteries, one vein) on a cross-sectional color Doppler image; a single umbilical artery (~1% of pregnancies) is the most common cord anomaly and is discussed as a soft marker in the abdominal/cord stage. Amniotic fluid volume is estimated (amniotic fluid index or single deepest pocket) — both oligohydramnios and polyhydramnios can be the first clue to an otherwise-missed structural or functional anomaly (renal, gastrointestinal, neuromuscular) and prompt a re-directed anatomic review.

When to escalate: targeted (Level II) ultrasound and fetal MRI

A standard scan is escalated to a targeted/detailed (Level II) anomaly scan, typically performed by maternal-fetal medicine, when any of the following are present:

• A suspected structural anomaly on the standard scan requiring more detailed characterization • Suboptimal visualization of one or more required structures (maternal obesity, fetal position, oligohydramnios, scarring) that cannot be resolved with repositioning or a repeat visit • Elevated a priori aneuploidy risk (abnormal serum/cfDNA screening, advanced maternal age, prior affected pregnancy) or multiple soft markers identified • Known teratogen exposure, pregestational diabetes, or a family history of a heritable structural anomaly

Fetal MRI is reserved for a smaller subset — most often suspected central nervous system anomalies (where its superior soft-tissue resolution and larger field of view add real diagnostic value over ultrasound, e.g., corpus callosum abnormalities, posterior fossa anomalies, cortical migration disorders), complex thoracic or abdominal masses affecting surgical planning, or cases where ultrasound views remain fundamentally limited by maternal habitus.

Detection rates are not a single number: population screening studies report overall major-anomaly detection anywhere from roughly 50% to over 90%, driven mainly by anomaly type (near-100% for anencephaly, much lower for some cardiac and skeletal lesions), maternal BMI, and center/operator experience.

What a normal scan does — and does not — guarantee

A completed, technically adequate anatomy scan with no structural anomalies and no soft markers substantially lowers — but does not eliminate — the likelihood of a major congenital anomaly or an underlying chromosomal condition. It should always be communicated as a screening result, contextualized alongside serum/cfDNA aneuploidy screening, not as a diagnostic guarantee of a structurally normal newborn.

When soft markers are present, current practice increasingly favors a combined risk approach — integrating the number and type of markers with prior aneuploidy screening results — over reflexive invasive testing for any single isolated marker. When a major anomaly is found, immediate steps typically include confirmatory imaging (targeted scan, sometimes fetal MRI or echocardiography), genetic counseling with diagnostic testing options (amniocentesis, karyotype/microarray), and referral to the relevant pediatric subspecialty team for delivery and postnatal management planning.