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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.

Two stacked transabdominal ultrasound frames with machine annotation showing a placenta partly separated from the uterine wall by a retroplacental collectionTransabdominal, two frames · Third trimester
source image carries labels

OB ultrasound · Placenta

Placental abruption on ultrasound (labelled composite)

Diagnosis

Placental abruption (retroplacental haematoma)

Normal against abnormal

The plane, and which way is up

Transabdominal, then transvaginal for the lower edge: the placenta as a homogeneous grey plate with a bright chorionic plate on the fetal side and the retroplacental space on the uterine side.

Look for, in order

  1. Where is it? Anterior, posterior, fundal, and its relation to the internal os: covering, reaching, or clear by more than 20 mm.
  2. The cord insertion: central, marginal or velamentous, best shown with colour Doppler.
  3. Its texture and thickness: lakes and calcification are common late; a thick heterogeneous placenta with a haematoma behind it is the picture of abruption, though abruption is a clinical diagnosis.
  4. In a previous caesarean with an anterior low placenta: loss of the clear zone, lacunae and bladder wall interruption suggest accreta.

What normal looks like

A placenta clear of the os, with a central cord insertion, of thickness roughly the weeks in millimetres; a low placenta at 20 weeks usually migrates and is re-checked at 32 weeks.

Trap. A full bladder or a contraction can create a false low-lying placenta; empty the bladder and look again.

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 collection between the placenta and the uterine wall
  • The placenta lifted away from the myometrium over part of its bed
  • Fetal parts beyond the placenta

What it means for the patient

Abruption is a clinical diagnosis: a tense tender uterus, pain, bleeding that may be concealed, and a fetal heart rate that is often the first thing to change. Ultrasound misses more than half of abruptions, because fresh blood is isoechoic with placenta; a normal scan never excludes it. When it does show, as here, the retroplacental collection separates the placenta from the wall. The scan's real job in a bleeding patient is to exclude praevia before anyone examines the cervix.

Framework: Cunningham et al., Williams Obstetrics, 26th edition (maternal physiology, labour mechanics, placental anatomy) (2022).

OBGynSpace Tutor

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