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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 at eight weeks showing two gestational sacs side by side within the uterus, each containing an embryo, separated by a membraneTransvaginal · 8 weeks

OB ultrasound · Multiple pregnancy

Early scan, larger than expected for dates

Diagnosis

Dichorionic diamniotic twin pregnancy

Normal against abnormal

The plane, and which way is up

Transvaginal or transabdominal at the earliest scan, looking at the membrane between the sacs where it meets the placenta.

Look for, in order

  1. Count the sacs and the embryos.
  2. Chorionicity: two separate placentas or a thick membrane with a triangular tongue of placental tissue (the lambda or twin-peak sign) means dichorionic; a thin membrane with a T-shaped insertion means monochorionic.
  3. Amnionicity in monochorionic twins: is there a dividing membrane at all?
  4. Label the twins by position and record it so every later scan follows the same twin.

What normal looks like

Chorionicity is best determined before 14 weeks; dichorionic twins have the lower risk. Monochorionic twins are scanned every two weeks from 16 weeks for twin-twin transfusion.

Trap. A single placental mass can hide two fused dichorionic placentas; use the membrane insertion, not the number of placental masses.

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

  • Two gestational sacs, each with an embryo
  • A thick inter-twin membrane with a triangular projection of chorion extending into its base from the placenta: the lambda (twin peak) sign

What it means for the patient

Chorionicity is the single most important fact about a twin pregnancy and it is easiest to establish before 14 weeks: two separate sacs, a thick membrane, and the lambda sign all mean dichorionic. A thin membrane with a T-shaped insertion means monochorionic, and a monochorionic pregnancy needs a different surveillance schedule from 16 weeks for twin-twin transfusion.

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

OBGynSpace Tutor

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