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OB ultrasound

33 studies11 challenges

Where is the pregnancy, how old is it, is it well: the questions each image answers, in the order the clinic asks them. Every image opens with how to read it; switch to Practice when you are ready to be tested.

Axial ultrasound of a fetal head at nineteen weeks showing the two lobes of the cerebellum and the fluid space behind them, with calipersTranscerebellar (modified axial) · 19+2 weeks

OB ultrasound · Anatomy scan

Anatomy scan, posterior fossa view

Diagnosis

Normal posterior fossa at the anatomy scan

Normal against abnormal

The plane, and which way is up

Transabdominal at 18 to 22 weeks, in standard planes: the transthalamic and transcerebellar planes of the head, the four-chamber heart, the abdominal circumference plane with the stomach and umbilical vein, the femur, the sagittal spine and the face in profile and coronal view.

Look for, in order

  1. Head: skull shape and ossification, the midline, the cavum septi pellucidi, the ventricles (atrium under 10 mm), the cerebellum and cisterna magna (under 10 mm), the nuchal fold.
  2. Face: the profile with the nasal bone, the lips in coronal view, the orbits.
  3. Spine: intact skin line and vertebral arches in sagittal, transverse and coronal planes.
  4. Heart: situs, four chambers of equal size, the crux, the outflow tracts crossing.
  5. Abdomen and limbs: stomach on the left, kidneys and bladder, a three-vessel cord, the long bones and the hands and feet.
  6. Then the placenta, the cervix, the fluid and the biometry.

What normal looks like

Each plane has a checklist of structures; a normal scan is a scan where every one was seen. Biometry within range for the dates; fluid normal; placenta clear of the os.

Trap. A structure not seen is not a structure that is normal. Record what could not be seen and bring her back.

Describe it in this order

  1. The plane and the approach (transabdominal or transvaginal)
  2. Location of the pregnancy: in the uterus or not
  3. Number: singleton or multiple, and chorionicity
  4. Viability: cardiac activity
  5. Dating or growth: the measurement that fits the stage
  6. Fetal anatomy, placenta, cord, fluid, cervix, as the stage allows
  7. Adnexa and free fluid
  8. Then, and only then, the impression

Findings

  • Normal dumbbell-shaped cerebellum with both hemispheres and the vermis
  • Normal cisterna magna, measured within the 2 to 10 mm range
  • Cavum septi pellucidi visible anteriorly

What it means for the patient

Three axial planes make up the routine head survey: transventricular (the ventricles), transthalamic (the biparietal diameter and head circumference) and transcerebellar (this one). An obliterated cisterna magna with a 'banana' cerebellum is the posterior fossa sign of open spina bifida, which is why this plane is part of every anatomy scan.

Framework: ISUOG, Practice guidelines: first-trimester and mid-trimester fetal ultrasound (2022).

OBGynSpace Tutor

It coaches the description; it does not read the image for you and will not name the diagnosis before you commit.