Midway through pregnancy, expecting parents reach one of the most critical clinical milestones: the week 20 anomaly scan. Often referred to as the mid-trimester structural scan, mid-pregnancy anatomy scan, or level 2 ultrasound, this detailed examination evaluates every major organ system, measures physical growth trajectory, and assesses placental health.

While many parents look forward to this appointment as an opportunity for a gender reveal ultrasound, its fundamental medical purpose is a thorough physical examination from head to toe.

What Does the 20-Week Anomaly Scan Check?

Performed typically between weeks 18 and 22 of pregnancy, the anomaly scan is a systematic evaluation led by a specialized sonographer or maternal-fetal medicine expert. The scan meticulously examines the following key structures:

1. Brain and Skull Architecture

The sonographer checks the shape and structure of the skull, verifying that the brain’s ventricles are normal in size. They examine the cerebellum (the back of the brain responsible for coordination) and the cisterna magna to screen for neural tube conditions such as spina bifida.

2. Facial Features and Profile

The ultrasound probe evaluates the upper lip and palate to check for cleft lip or palate. The profile is checked to confirm the presence of a nasal bone and appropriate facial proportions.

3. Cardiac Chamber and Vessel Flow

The heart is one of the most complex organs evaluated. The sonographer inspects the four distinct chambers, verifies the rhythm, and traces the major outflow tracts (the aorta and pulmonary artery). If an irregularity is noted, a specialized fetal echocardiogram may be ordered for a higher-resolution assessment of fetal cardiac hemodynamics.

4. Spine, Abdomen, and Internal Organs

  • Spine: Swept along both longitudinal and axial planes to ensure vertebrae are fully closed and skin coverage is intact.
  • Stomach & Kidneys: The presence of a fluid-filled stomach confirms the baby is swallowing amniotic fluid properly. The sonographer also inspects both kidneys and checks that the urinary bladder fills and empties.
  • Abdominal Wall: Confirms that the abdominal wall is completely closed around the umbilical cord insertion site.

Key Biometric Growth Measurements

During the scan, four standard fetal biometrics are recorded to track growth against standard charts and confirm your due date calculator estimates:

                  +-----------------------------------+
                  |  20-WEEK BIOMETRIC MEASUREMENTS   |
                  +-----------------------------------+
                                    |
     +-----------------+------------+------------+------------------+
     |                 |                         |                  |
     v                 v                         v                  v
[ BPD ]             [ HC ]                    [ AC ]             [ FL ]
Biparietal        Head Circumference    Abdominal Circumference  Femur Length
Diameter          (Overall skull size)  (Torso & soft tissue)   (Thigh bone length)
(Ear-to-ear width)

Combining these four parameters yields an Estimated Fetal Weight (EFW), helping physicians identify potential growth restriction or accelerated fetal size early in the second trimester.

Placenta, Cord, and Amniotic Fluid Assessment

In addition to examining fetal anatomy, the anomaly scan evaluates the life-support structures maintaining the pregnancy:

  • Placental Location: Determines whether the placenta is positioned on the anterior (front), posterior (back), or fundal (top) wall of the uterus. Importantly, it screens for placenta previa—a condition where the placenta partially or completely covers the internal cervical os.
  • Umbilical Cord Insertion: Confirms a healthy three-vessel cord (two arteries, one vein) and evaluates its connection to the placenta.
  • Amniotic Fluid Index (AFI): Assesses whether fluid levels are within an optimal range, avoiding oligohydramnios (too little fluid) or polyhydramnios (excess fluid).

Understanding Your Results

In the majority of cases, the anomaly scan provides reassuring news, confirming typical structural development. However, if the scan identifies a “soft marker” (such as an isolated echogenic intracardiac focus or minor renal pelvis dilation) or a structural variation, your obstetrician will discuss next steps clearly. This may include scheduling a targeted high-resolution scan, consulting with a specialist, or performing follow-up non-invasive prenatal testing (NIPT) to ensure comprehensive care for both parent and baby.

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