The membranes are separated in early gestation, accounting for the appearance of the amniotic sac. After approximately 14 weeks gestation, these membranes fuse and are indistinguishable as separate entities. Rarely, a chorioamniotic separation can occur later in gestation. It can be focal or extensive, with the amniotic membrane becoming either free-floating or adherent to the fetus.
Most reported cases occur after invasive intra-uterine procedures such as:
- previous amniocentesis: small degrees of CAS are frequently present after amniocentesis
- hysterotomy 5
Some cases occur spontaneously which is then termed a primary chorioamniotic separation.
- increased rates of underlying fetal chromosomal and developmental abnormalities have been associated with sporadic cases 5
A chorioamniotic separation is usually sonographically detected as a visible free-floating or adherent membrane surrounding the fetus. The separation can extend throughout the entire uterine cavity up to the base of the umbilical cord and over the surface of the placenta.
Sonographic detection of a small chorioamniotic membrane separation especially before 14 weeks gestation is considered a benign incidental finding 5,7
Knowledge of potential complications is however useful. A careful search for aneuploidy and other developmental anomalies is often recommended if a spontaneous chorioamniotic separation is detected after 14 weeks.
Recognized complications following a large separation include 4:
- in utero fetal death
- umbilical cord complications
- pre-term delivery
- amniotic band formation
Considerations for ultrasound appearances include:
- amniotic band going through the periphery: can be also a complication of CAS
- subchorionic hemorrhage dissecting around the endometrial cavity 2
- 1. Elsayes KM, Trout AT, Friedkin AM et-al. Imaging of the placenta: a multimodality pictorial review. Radiographics. 29 (5): 1371-91. doi:10.1148/rg.295085242 - Pubmed citation
- 2. Trop I, Levine D. Hemorrhage during pregnancy: sonography and MR imaging. AJR Am J Roentgenol. 2001;176 (3): 607-15. AJR Am J Roentgenol (full text) - Pubmed citation
- 3. Levine D, Callen PW, Pender SG et-al. Chorioamniotic separation after second-trimester genetic amniocentesis: importance and frequency. Radiology. 1998;209 (1): 175-81. Radiology (abstract) - Pubmed citation
- 4. Lewi L, Hanssens M, Spitz B et-al. Complete chorioamniotic membrane separation. Case report and review of the literature. Fetal. Diagn. Ther. 19 (1): 78-82. doi:10.1159/000074266 - Pubmed citation
- 5. Barak S, Leibovitz Z, Degani S et-al. Extensive hemorrhagic chorion-amnion separation after second-trimester genetic amniocentesis. J Ultrasound Med. 2003;22 (11): 1283-8. J Ultrasound Med (full text) - Pubmed citation
- 6. Bromley B, Shipp TD, Benacerraf BR. Amnion-chorion separation after 17 weeks' gestation. Obstet Gynecol. 1999;94 (6): 1024-6. - Pubmed citation
- 7. Graf JL, Bealer JF, Gibbs DL et-al. Chorioamniotic membrane separation: a potentially lethal finding. Fetal. Diagn. Ther. 12 (2): 81-4. - Pubmed citation