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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.

Transabdominal ultrasound in the second trimester showing parts of two fetuses in one field, bright bony structures of each visibleTransabdominal · Second trimester

OB ultrasound · Multiple pregnancy

Second-trimester scan, uterus large for dates

Diagnosis

Twin pregnancy in the second trimester

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 fetal bodies in one field of view
  • Bony structures of each fetus visible at once
  • The inter-twin membrane must be sought separately to establish chorionicity

What it means for the patient

Once the pregnancy is beyond the first trimester, two fetuses in one field are easy to see and chorionicity is hard to call: the membrane thins, the placentas may fuse, and the lambda sign fades. That is why the chorionicity from the early scan is written on the front of the chart and never re-derived later. Growth is then plotted for each twin on its own, every four weeks for dichorionic and every two for monochorionic twins.

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.