Linear Echoes in Amniotic Fluid

Anne Kennedy, MD

DIFFERENTIAL DIAGNOSIS

Common

Synechiae
Dichorionic Diamniotic Twins
Monochorionic Diamniotic Twins
Chorioamniotic Separation
Placental Abruption, Old
Uterine Septum
Placental Cysts

Less Common

Amniotic Band Syndrome
Circumvallate Placenta

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Single or multiple gestation?
- If multiple, inter-twin membrane is most likely cause of a linear echo in the amniotic fluid
Does linear echo cross cavity from side to side?
- Placental edge to placental edge → circumvallate placenta
- Uterine wall to uterine wall → synechia
Does linear echo parallel wall of uterine cavity?
How do linear echoes relate to placenta?
How do linear echoes relate to fetus?
Is there a history of intervention?
- Prior uterine instrumentation
  - D&C, myomectomy, metroplasty
- Procedure performed during current pregnancy?
  - Amniocentesis, amnioreduction, intrauterine transfusion, laser therapy or radiofrequency ablation

Helpful Clues for Common Diagnoses

Synechiae
- Shelf or band-like structure which does not restrict fetal movement
- Extra-amniotic: Fetal membranes wrap over synechiae
- Straight, bulbous free edge with thinner sheet extending to endometrial surface
- Y-shaped notch at endometrial base, created by membranes separating at endometrial margin
- Placenta can abut or even wrap around synechia
- Color Doppler may demonstrate flow within synechiae
- In first trimester may cause distortion of gestational sac shape
- May no longer be visible in 3rd trimester due to rupture or compression
Dichorionic Diamniotic Twins
- Thick echogenic chorion completely surrounds each embryo
- “Twin peak” sign: Wedge of chorionic tissue extending into base of inter-twin membrane
- Two fetuses in separate chorionic sacs
  - Two amniotic sacs with two yolk sacs
- Dichorionic membrane actually separates two amniotic cavities (i.e., extra-amniotic)
Monochorionic Diamniotic Twins
- Two fetuses in single chorionic sac containing two amniotic sacs
- Thin inter-twin membrane formed by two layers of amnion without interposed chorion
- No “twin peak”
- Twins must be same gender
Chorioamniotic Separation
- Persistent unfused amnion and chorion > 16 wks
- Amniotic membrane separate from uterine wall
  - Complete: Attached only at placental cord insertion site
  - Incomplete: Unattached around part of the uterine cavity, the commonest form
- May be primary non-fusion
  - Look for signs of aneuploidy
- May occur secondary to amniocentesis or fetal intervention
  - Increases risk of membrane rupture in twins → functional monoamniotic state → ↑ risk of cord entanglement
Placental Abruption, Old
- Hypoechoic blood clot near or behind placenta
- Marginal (most common), retroplacental or preplacental
  - Marginal: Bleed at edge of placental disc, dissects between chorion and uterine wall
  - Retroplacental does not cause confusion for linear echoes in amniotic fluid as located between placenta and myometrium
- Preplacental abruption is rare
  - Hematoma on fetal surface of placenta
  - Clot may compress cord if close to insertion site
- Subacute
  - May contain fluid-fluid level, septations common
- Old
  - Liquefying blood, eventually sonolucent and may mimic amniotic fluid
- Hemorrhage can dissect under chorionic membrane
  - Clot seen at a distance from placenta
  - Look in front of cervical os
- Intraamniotic blood common → echogenic fluid → echogenic fetal bowel from swallowed blood
- In twins, rarely, hematoma dissects between membranes
  - When old may appear as fluid-filled mass between membranes → increased linear interfaces in amniotic fluid
Uterine Septum
- Midline, arising from fundus
  - In first trimester use 3D to create coronal images and confirm location, assess fundal contour
- May be fibrous or composed of myometrium
- Thicker than synechiae
- Creates two distinct endometrial cavities
Placental Cysts
- Chorionic cysts are simple cysts on fetal placental surface
- Often near cord insertion site
- If multiple may appear to create linear echoes in amniotic fluid
- Curvilinear shape and relationship to placenta indicate etiology
- If large, or if hemorrhage occurs, may compress cord

Helpful Clues for Less Common Diagnoses

Amniotic Band Syndrome
- Entrapment of fetal parts by disrupted amnion
- Amniotic band in contact with deformity, extends to uterine wall
- Bands in amniotic fluid appear as multiple thin membranes
- No flow in band on Doppler evaluation
Circumvallate Placenta
- Placental margin elevated off uterine wall
- Scanning parallel to edge → “marginal shelf”
- Scanning longitudinally → “curled lip” of placental margin