The 18–22 week fetal anatomy survey (level 1 ultrasound) is one of the most important scans in obstetrics. Every structure you document — or fail to document — can have clinical implications. This guide walks through the complete anatomy survey with key images, measurements, and what to look for.

When It's Performed

Standard timing: 18–22 weeks gestational age
Optimal: 18–20 weeks for anatomy + 20–22 weeks for cardiac detail
Earlier (16 weeks): limited visualization, may require repeat
Later (> 24 weeks): technically adequate, but earlier detection allows more counseling options

Equipment and Preparation

Curved array 3–5 MHz for most patients; phased array through ribs if needed
Transabdominal is primary; transvaginal available for cervical length, low-lying placenta, or limited transabdominal views
Patient: full or partially full bladder is helpful but not required (for most structures)

Fetal Measurements (Biometry)

Document all four standard biometric measurements:

Measurement	Plane	How to Measure
Biparietal Diameter (BPD)	Axial brain — thalami visible, midline echo, cavum septi pellucidi	Outer edge to inner edge of skull
Head Circumference (HC)	Same axial plane as BPD	Outer perimeter of skull (ellipse or manual trace)
Abdominal Circumference (AC)	Transverse abdomen — stomach bubble + umbilical vein junction	Outer perimeter
Femur Length (FL)	Longest bone length	Outer edge to outer edge of ossified diaphysis (not epiphysis)

Calculate estimated fetal weight (EFW): Use a standard formula (Hadlock 4-parameter is most common). Document EFW with the percentile for gestational age.

Amniotic Fluid: Document AFI or largest vertical pocket (MVP). Note if oligohydramnios or polyhydramnios.

Placental location: Anterior / posterior / fundal / right lateral / left lateral. Note relationship to the internal os — document if placenta appears low-lying (< 2 cm from os).

Head and Brain

Axial views (at three levels):

1. Transthalamic plane (BPD/HC plane):

Thalami (oval midline structures)
Cavum septi pellucidi (CSP) — a small box-like anechoic space anterior to the thalami
Cerebral peduncles
This is the BPD/HC measurement plane

2. Transventricular plane:

Lateral ventricles — measure the atrium of the posterior lateral ventricle
Normal: ≤ 10 mm (inner-to-inner at the atrium, at the level of the choroid plexus)
10 mm = ventriculomegaly
Choroid plexus fills the atrium and should be touching the medial wall — note if choroid plexus is "dangling" (hanging free in a dilated ventricle)

3. Transcerebellar plane:

Cerebellum — measure transverse diameter (TD)
Cisterna magna — measure AP diameter (normal 2–10 mm)
Nuchal fold — only measured at 15–20 weeks: normal < 5 mm at this level
Vermis should be intact between the two cerebellar hemispheres

Posterior fossa:

Banana sign: flattened cerebellum (myelomeningocele/spina bifida)
Lemon sign: scalloping of the frontal bones (also spina bifida)
Dandy-Walker: enlarged cisterna magna, absent vermis

Facial features:

Profile: nose, lips, chin, forehead
Lip: coronal view of the upper lip and nose — document if both nostrils visible, intact lip
Orbits: axial view with both orbits — measure OOD (outer orbital diameter) and IOD (inner orbital distance)

Spine

Goal: Exclude spina bifida (open neural tube defect).

Sagittal spine: complete spine from cervical to sacrum in one or multiple images
Transverse spine: axial views at cervical, thoracic, lumbar, and sacral levels
Normal: intact skin line over the spine, intact posterior elements (two ossification centers seen as echogenic structures flanking the spinal canal)
Open NTD: disrupted posterior elements, splaying of the dorsal ossification centers, soft tissue defect through the skin

Heart

The heart is the most complex part of the anatomy survey and the most common site of missed anomalies.

Minimum required views (per AIUM guidelines):

Four-chamber view:
- Heart occupies ~1/3 of the chest
- Axis: pointing left (toward left chest wall, 45–60° from midline)
- Four chambers roughly equal size (RA ≈ LA; RV ≈ LV)
- Intact interventricular and interatrial septa
- Atrioventricular valves open and close normally
- Pulmonary veins entering the left atrium (ideally 2 visible)
LVOT (Left ventricular outflow tract):
- Aorta arises from the left ventricle
- Septal continuity between the interventricular septum and the aortic root (no VSD at the LVOT)
RVOT (Right ventricular outflow tract):
- Pulmonary artery arises from the right ventricle
- The great vessels cross — aorta and pulmonary artery should cross each other
- Great vessel diameters approximately equal
Three-vessel view:
- Transverse view at the level of the great vessels
- Three vessels visible in order from right to left: SVC, aorta, main pulmonary artery
- PA > Ao > SVC in size
Three-vessel trachea view:
- Trachea seen as a ring posterior to the three vessels
- Ductus arteriosus visible connecting PA to descending aorta
- V-sign or "ductal arch" forms

Cardiac position and axis: "Dextrocardia" = heart points right. "Mesocardia" = heart points midline. Both are abnormal and warrant further evaluation.

Chest and Abdomen

Lungs: Echogenic, equal bilaterally. Lung mass displacing the heart = CPAM or sequestration concern.

Diaphragm: Document intact diaphragm. Absence or herniation = CDH (congenital diaphragmatic hernia) — look for bowel or stomach in the chest.

Stomach: Must be visible. A fluid-filled stomach in the left upper quadrant confirms swallowing function and esophageal patency. Absent stomach = esophageal atresia concern (or too early / timing issue).

Abdominal circumference plane: Umbilical vein courses through the liver and is visible in the AC plane. The stomach bubble should be visible.

Abdominal wall: The umbilical cord inserts normally at the abdominal wall. Bowel outside the abdomen = omphalocele (covered by membrane, cord inserts at apex) vs. gastroschisis (uncovered, cord inserts to the side of the defect).

Kidneys: Both kidneys visible. Normal echogenicity (not hyperechoic). Normal pelvic AP diameter: < 4 mm before 20 weeks, < 7 mm at 20–33 weeks.

Bladder: Visible as an anechoic structure in the fetal pelvis. Absent bladder = renal agenesis concern (combined with oligohydramnios).

Extremities and Gender

Long bones: Document all four limbs. Measure bilateral femur, humerus (optional per protocol). Significant shortening or bowing = skeletal dysplasia concern.

Hands and feet: Digits visible. Note if hands are open or clenched (clenched fists can indicate trisomy 18). Foot: look for clubfoot (talipes equinovarus — foot turns inward and is abnormally positioned).

Fetal sex: Document if clearly seen and if the patient wishes to know. "Fetal sex not determined" is acceptable if ambiguous. Note if patient requested not to be told.

Cervical Length (If Protocol Includes It)

For patients with shortened cervix risk (prior preterm delivery, uterine anomaly):

Transvaginal measurement
Empty bladder
Measure from the internal os to the external os in sagittal
Normal: ≥ 25 mm at 18–24 weeks
< 20 mm = short cervix requiring management discussion

Images Checklist

Per AIUM guidelines for the standard OB ultrasound, minimum required images include:

Fetal number and presentation
Biometry: BPD, HC, AC, FL (with measurements labeled)
Placental location
AFI or MVP
Brain: BPD/HC plane, transventricular, posterior fossa
Spine sagittal and transverse
Heart: 4-chamber, LVOT, RVOT (or 3-vessel view)
Stomach
Abdominal wall (umbilical cord insertion)
Kidneys bilaterally
Bladder
Long bones bilaterally (at minimum femur)
Fetal face (profile and coronal lip)
Cervix (if indicated)

Document any structure not seen and why: "Fetal position limited evaluation of [structure] — additional views obtained with position change / transvaginal imaging / follow-up recommended."

SonoBuddy includes first and second trimester measurement references in the OB Measurements section — including biometry, ventricular width, and cisterna magna normal values.

Second Trimester Anatomy Scan: Complete Protocol and What to Document