Chorioamniotic Separation (CAS)

Definition - What does Chorioamniotic Separation (CAS) mean?

Chorioamniotic separation (CAS) is a condition in which the innermost covering of the embryo called amnion separates from the outermost covering called chorion after the 16th week of pregnancy, leading to miscarriage. It is a rare condition and has an association with fetal and chromosomal abnormalities.

FertilitySmarts explains Chorioamniotic Separation (CAS)

The partition of the chorion and amnion before 14 weeks of pregnancy is normal. The amnion and chorion typically unite between 14 and 16 weeks. Separation of the two layers that persists after the 16th week is not normal and can lead to several life-threatening complications.

CAS most commonly occurs after invasive uterine procedures during pregnancy such as retrieval of the amniotic fluid from the sac around the fetus called amniocentesis, fetal blood sampling, or fetal surgery.

In some cases, CAS can also occur spontaneously without any known trigger. Whatever the cause be, CAS is typically diagnosed on a routine ultrasound examination, or on an ultrasound done when the woman shows up with one of the following complications:

  • Miscarriage
  • Death of the fetus inside the uterus
  • Umbilical cord complications such as cord compression
  • Rupture of the amnion with the formation of fibrous amniotic bands floating in the bag of waters that can entangle the fetus and constrict its blood supply, leading to fetal death
  • Emergent C-section due to fetal distress leading to premature delivery

In cases where the fetus is still alive, the doctor will monitor a CAS pregnancy very closely with serial non-stress tests and ultrasound evaluation (including assessment of fetal growth and early detection of any complications). If there’s any evidence of fetal distress, delivery is performed in the early term period.

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