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OB ultrasound

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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.

Ultrasound of a fetal profile at thirteen weeks with calipers across the fluid at the back of the neck and the nasal bone markedMid-sagittal profile, magnified so the head and upper thorax fill the image · 13+3 weeks

OB ultrasound · First-trimester screening

Combined first-trimester screening scan

Diagnosis

Normal nuchal translucency and nasal bone at 13 weeks

Normal against abnormal

The plane, and which way is up

Mid-sagittal of the fetus at 11 to 14 weeks (CRL 45 to 84 mm), the fetus filling most of the screen, in a neutral position with the nasal bone, the tip of the nose and the palate in view and the amnion separate from the skin of the neck.

Look for, in order

  1. The nuchal translucency: the black fluid space between the skin and the soft tissue over the cervical spine, measured at its widest with the callipers on the inner borders.
  2. The nasal bone: a bright line inside the skin line of the nose, brighter than the skin above it.
  3. Other early markers where trained: tricuspid flow, ductus venosus, the intracranial translucency.

What normal looks like

A nuchal translucency below the 95th centile for the CRL (roughly under 3 mm at 12 weeks) and a nasal bone present. The measurement is combined with maternal age and serum markers; it is a risk, not a diagnosis.

Trap. Measuring the amnion instead of the skin, a flexed neck, or a poorly magnified image all change the reading by the fractions of a millimetre that matter.

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

  • Nuchal translucency 2.1 mm, within normal limits for this crown-rump length
  • Nasal bone present
  • Facial angle measured; a Fetal Medicine Foundation certified image

What it means for the patient

Nuchal translucency is measured between 11+0 and 13+6 weeks, in a mid-sagittal plane with the fetus neutral, calipers on the inner borders of the fluid space, taking the largest of several measurements. The number is meaningful only against the crown-rump length and only within a screening programme that combines it with maternal age and serum markers.

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.