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Radiology
Dichorionic Diamniotic Twin Gestations
Built from Obstetric Imaging
What’s inside
8 sections · 83 slides
Overview
- What you will be able to do after this
- Why anyone bothers with this label
What the words mean
Chorion, amnion, and why one of the two words is spare
- Chorion and amnion: the two bags
- Dichorionic, in plain words
- Diamniotic, in plain words
- One of those two words is redundant
- The three twin arrangements
- 33.9
- Reading those numbers
How this kind of twin is made
One egg or two, and why the timing of a split matters
- Twins start in one of two ways
- Monozygotic and dizygotic
- How the two routes lead to two placentas
- Two eggs almost always means two placentas
- How common is each kind of twinning
- For identical twins, timing decides everything
- The split that comes early keeps the placentas apart
- A trap: same sex does not mean shared placenta
Suspecting twins, then proving it
What raises the question, and what can actually answer it
- What makes you suspect more than one baby
- What the first scan has to answer
- Two fetal heads in one uterus
The ultrasound signs
Sacs, placentas, membranes and the lambda sign
- Earliest window: 6 to 10 weeks
- Two sacs, each with its own embryo and yolk sac
- Later on, check three things
- Working through a scan for chorionicity
- The thick dividing membrane
- The twin peak, better known as the lambda sign
- >97%
- What those two numbers mean
- Lambda sign with a thick intertwin membrane
- Two placental masses with a thick dividing membrane
- Close view of the lambda wedge
- Two placentas, two cord insertions
- Different sexes settles it
- How well does the whole package perform
- Scan early: the accuracy fades with time
- Early scan versus late scan
- Measuring the membrane in mid-pregnancy
- The membrane cut-offs
- Thick membrane versus thin membrane
- Layers of the dividing membrane under the microscope
Telling the two twin types apart
Dichorionic versus monochorionic diamniotic
- What you are really deciding between
- What a dichorionic pair looks like
- What a monochorionic pair looks like
- Lambda sign or T sign
- Why the answer changes the care plan
Using ultrasound through the pregnancy
Screening, anatomy, cervix, growth and delivery
- What twin pregnancies are prone to
- What serial ultrasound is looking for
- When each scan happens
- Nuchal translucency: measuring each baby separately
- Why this measurement works well in twins
- Measuring nuchal translucency
- Screening from a blood sample
- The blunt caveat for twins
- 99.7%
- Reading the singleton benchmark
- When the test fails to give an answer
- NIPT numbers in dichorionic twins
- 89%
- What a positive predictive value tells a patient
- Why these NIPT figures are shakier than they look
- The anatomy survey
- Measuring the cervix to predict early birth
- What the cervical length threshold is
- Cervical length measured on transvaginal ultrasound
- The uncomfortable gap between predicting and preventing
- Growth problems in twins
- How often to repeat the growth scan
- What the evidence base does and does not say
- When to think about prenatal diagnosis
- Doppler: helpful only when something is wrong
- Uncomplicated versus complicated twins
- Ultrasound in the delivery room
- After the babies are born
Take-home messages
What the referring clinician needs from the scan
- What the referring physician needs to know
- Key points: the names
- Key points: what to look for
- Key points: what ultrasound is for
- Five things worth memorising
- References
- References (continued)
- References (continued)
- Suggested readings
- Obstetric Imaging: Fetal Diagnosis and Care, 2nd Edition