CHORIOAMNIOTIC SEPARATION (CAS) (1-7)

 

Sonographic characteristics of membrane

Relationship to Fetus / Placenta

Timing / Etiology

* Dissection of fluid between the amnion and chorion up to the base of the umbilical cord.
* Elevated membrane (amnion) appears thinner than the chorion.

* No fetal distress
* Extension of lucent area over fetal surface of placenta with cessation at the cord (unlike sub-chorionic hemorrhage)
* Surface of placenta under lucent area is regular and smooth (unlike placental lakes).

* Amnion is a thin membrane separated from chorion by an anechoic space, which obliterates at 14-16 weeks gestation.
* Non-fusion >16wks is diagnostic of chorioamniotic separation which is usually due to hemorrhage:
  - traumatic
  - amniocentesis.

 
  • Classification:
    • Complete – amnion separated from the chorion on at least three sides of the gestational sac (3).
    • Partial non-fused membranes (limited form) (4,5).
  • The separation required between the two membranes to define CAS has been reported as 10 mm (1,2), 3 mm (4,5) or 1 mm (7).
  • There have been reported cases of CAS beyond 14 weeks that have an association with chromosomal aneuploidy.
  • The highest rates of aneuploidy were observed in studies, which only included major separation.
  • Bronshtein and Zimmer (1) describe major CAS as being associated with oligohydramnios in the early second trimester (all nine cases were interrupted within 8 days of the diagnosis).
  • Abboud and coworkers (6) describe a case of CAS at 14 weeks associated with oligohydramnios at the initia scan, however the fluid normalized when the membranes were found to be fused 5 weeks later. The fetus had Down syndrome.
  • Isolated CAS was associated with aneuploidy more frequently when the separation between the membranes was large (>10 mm), and complete (6).
  • CAS that occurs after amniocentesis is usually of the limited form with no specific consequences on the pregnancy (7).

 

Ref

Population studied

CAS (n)

Aneuploidy (n)

Isolated CAS

 

 

 

 

Total (n)

With aneuploidy

1

7000 ultrasound

9

4

0

0

2

Not stated

5

4

2 (including only major CAS)

1

3

89,700 ultrasound

15

3

0

0

4

492 high risk fetuses

60

14

18 (including minor and partial CAS)

1

5

1950 amniocentesis

25

7

20 (including minor and partial CAS)

2

Adapted from reference (6)

 

 

 

 

 

Video clip of Chorioamniotic Separation

 

 

 

 

 

 

REFERENCES

 

1.      Bronhstein M, Zimmer E. Oligohydramnios with amnio-chorionic separation at 15-16 weeks of gestation. Prenat Diagn 1995;15:161-164.

2.      Appelman Z, Zalel Y, Fried S et.al. delayed fusion of amnion and chorion: a possible association with trisomy 21. Ultrasound Obstet Gynecol 1998;11:303-305.

3.      Bromley B, Shipp TD, Benacerraf BR. Amnion-chorion separation after 17 weeks of gestation. Obstet Gynecol 1999;94:1024-1026.

4.      Ulm B, Ulm MR, Bernaschek G. Unfused amnion and chorion after 14 weeks of gestation: associated fetal  structural and chromosomal abnormalities. Ultrasound Obstet Gynecol 1999;13:392-395.

5.      Montero JJ, Ortega S, Vasquez C et.al.  Delay of chorioamniotic fusion: relation to chromosomal anomalies. Prenat Diagn 2000;20:517-525.

6.      Abboud P, Mansour G, Zejli Aet.al. Chorioamniotic sepatation after 14 weeks gestation associated with trisomy 21. Ultrasound Obstet Gynecol 2003;22:94-100.

7.      Levine D, Callen PW, Pender SG et.al. Chorioamniotic separation sfter second trimester genetic amniocentesis: importance and frequency. Radiology 1998;209:175-181.