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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.

Ultrasound of a fetus in sagittal section at twelve weeks with the crown-rump length marked by calipersTransabdominal, mid-sagittal · 12+3 weeks

OB ultrasound · Dating

Twelve-week scan, sagittal

Diagnosis

Normal 12-week fetus, correctly measured crown-rump length

Normal against abnormal

The plane, and which way is up

Sagittal to the embryo or fetus: the crown-rump length is the longest straight line from the top of the head to the bottom of the rump, in a neutral (not flexed, not extended) position, excluding the yolk sac and the limbs.

Look for, in order

  1. Find the midline sagittal plane of the embryo: the whole length visible, the spine along one edge.
  2. Place callipers on the outer edges of the crown and the rump; take the best of three.
  3. Read the gestational age from the CRL chart; the measurement is accurate to about 5 days in the first trimester.
  4. Check that the sac, the yolk sac and cardiac activity are as expected for that age.

What normal looks like

A CRL of about 10 mm at 7 weeks, 17 mm at 8 weeks, 45 to 55 mm at 12 weeks; heart rate 110 to 160 after 6.5 weeks. A first-trimester CRL date that differs from the menstrual date by more than 5 to 7 days re-dates the pregnancy, and the date is then fixed.

Trap. A flexed or extended embryo, or a line that includes the yolk sac, gives a wrong length; after 14 weeks the CRL is unreliable and head and femur measurements take over.

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

  • Fetus in a true mid-sagittal plane, neutral position, neither flexed nor hyperextended
  • Crown-rump length measured from the top of the head to the rump, excluding the yolk sac and limbs

What it means for the patient

Correct technique matters more than the number: a mid-sagittal plane, a neutral neck, the whole fetus horizontal and filling the image. A flexed fetus under-measures; a hyperextended one over-measures. The same image, in the same window, is where nuchal translucency is measured.

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

OBGynSpace Tutor

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