Chorioamniotic separation

Chorioamniotic separation is an intrauterine event that can occur in pregnancy and is characterised by separation of the placental (chorion) and fetal (amnion) membranes.

Epidemiology

Associations

• increased rates of underlying fetal chromosomal and developmental abnormalities have been associated with sporadic cases ⁵

Pathology

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.

Aetiology

Most reported cases occur after invasive intrauterine procedures such as:

• previous amniocentesis: small degrees of chorioamniotic separation are frequently present after amniocentesis

• hysterotomy ⁵ 

Some cases occur spontaneously which is then termed "primary chorioamniotic separation".

Radiographic features

Antenatal ultrasound

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 separation before 14 weeks gestation is a normal and benign incidental finding ^5,7. A careful search for aneuploidy and other developmental anomalies is recommended if a spontaneous chorioamniotic separation is detected after 14 weeks.

Treatment and prognosis

Complications

Recognised complications following a large separation include ⁴:

• miscarriage

• fetal death in utero

• umbilical cord complications

• pre-term delivery

• amniotic band formation

Differential diagnosis

Considerations for ultrasound appearances include:

• amniotic band going through the periphery: can be also a complication of chorioamniotic separation

• subchorionic haemorrhage dissecting around the endometrial cavity ²

See also

• intrauterine membrane in pregnancy