← All decks
Radiology
Chorionicity of Multiple Gestations
Built from Obstetric Imaging
What’s inside
6 sections · 65 slides
Overview
- What this topic is about
The words, in plain English
Zygosity, chorionicity, amnionicity - three questions that sound alike and are not.
- Twins are not rare
- Two completely different ways to make twins
- How fraternal and identical twins begin
- Zygosity answers a different question
- Chorionicity: counting placentas
- Amnionicity: counting sacs
- Twins lying in separate sacs
- The three labels you will actually write
- The three types on one branch
Where each type comes from
Two eggs give one answer. One egg gives three, and the calendar decides which.
- Dizygotic twins: the simple case
- What makes two eggs drop at once
- Background rate differs between populations
- Monozygotic twins: the calendar decides
- The splitting clock
- The splitting clock, spelled out
- ~1/3
- One placenta almost always means one egg
- How often does each happen
Why the answer matters
Everything that makes twins dangerous beyond prematurity traces back to a shared placenta.
- Every twin pregnancy carries extra risk
- Sharing a placenta is the dangerous part
- What one shared placenta can cause
- Twin-to-twin transfusion: what goes wrong
- Twin-to-twin transfusion by the numbers
- When one twin dies, the other is in danger
- Stillbirth: monochorionic versus dichorionic
- Monoamniotic twins: the cords tangle
- Two rarer monochorionic problems
Making the diagnosis with ultrasound
Count sacs early, read the membrane late, and know what each sign is worth.
- Why not just look at the placenta afterwards
- The dividing membrane at delivery
- The first trimester is the best window
- The first-trimester routine
- Yolk sacs: a rule that has been dropped
- Settle amnionicity at the same visit
- What to look for, type by type
- Ultrasound Characteristics of Chorionicity and Amnionicity
- Chorionicity and amnionicity at a glance
- Missed the early window? Work systematically
- The twin peak, or lambda, sign
- A thick membrane with a wedge at its base
- >97%
- The T sign
- A hairline membrane meeting the placenta square-on
- 100%
- The two signs side by side
- How accurate is the whole approach
- Later in pregnancy the signs fade
- The two late fallbacks
- The trap: same sex and one placental mass
- One placental mass, two cords
- Measuring and counting the membrane
- A thick dividing membrane under the microscope
- A thin dividing membrane under the microscope
- Thick versus thin, in one line
- When the membrane cannot be seen: the stuck twin
- Stuck twin or monoamniotic? Move the mother
Key points
What has to survive from this topic.
- Key points: the words and the timing
- Key points: the two signs
- Traps worth remembering
- References
- References (continued)
- References (continued)
- References (continued)
- References (continued)
- Obstetric Imaging: Fetal Diagnosis and Care, 2nd Edition