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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.

Transvaginal ultrasound of the cervix in pregnancy with calipers along the closed cervical canal, part of the fetal head at the upper rightTransvaginal, sagittal cervix · 22 weeks

OB ultrasound · Cervix

Transvaginal cervical length at twenty-two weeks

Diagnosis

Normal cervical length at 22 weeks

Normal against abnormal

The plane, and which way is up

Transvaginal, sagittal, with an empty bladder and the probe withdrawn until the cervix is not compressed, the internal and external os both in view, the canal running between them.

Look for, in order

  1. Find the internal os (where the canal meets the amniotic cavity) and the external os.
  2. Measure the closed length of the canal in a straight line, three times over 3 to 5 minutes, and take the shortest.
  3. Look for funnelling of the membranes into the canal and for sludge in the fluid above it.

What normal looks like

A cervical length of 25 mm or more in the mid-trimester. Under 25 mm before 24 weeks in a singleton is short and is treated (vaginal progesterone, and cerclage in selected histories).

Trap. A full bladder or probe pressure lengthens the cervix and hides funnelling; a curved canal measured straight is under-read.

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

  • A closed cervical canal measured from internal to external os
  • Length over 40 mm, normal
  • No funnelling of the internal os

What it means for the patient

A short cervix (25 mm or less before 24 weeks) predicts preterm birth and is the trigger for vaginal progesterone, and in selected histories a cerclage. Technique: empty bladder, transvaginal probe, the whole canal in view, minimal pressure, measured over a few minutes because the cervix changes shape.

Framework: SOGC, Guideline: antenatal corticosteroids and management of preterm prelabour rupture of membranes (2018).

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