FIRST TRIMESTER PREGNANCY FAILURE

- THE EMPTY AMNION

• From 6.5 to 10 weeks, the CRL of the embryo is similar to the mean diameter of the amniotic cavity, and both have equal linear rates of growth.
• Mean diameter of amniotic cavity is ±10% greater than length of embryo (1).
• By the time the amniotic cavity is sonographically detectable, an embryo of similar length should also be visualized (2).
• An amniotic sac without an embryo = "empty amnion" and suggests an abnormal pregnancy (2).

• Cause of empty amnion is not clear. Either no embryo has formed or the embryo resorbed after its death leaving an empty amniotic cavity.
• After fetal death the amniotic cavity may continue to enlarge.

REFERENCES

1. Horrow MM. Enlarged amniotic cavity: A new sonographic sign of early embryonic death. AJR 1992:158:359-362.
2. McKenna KM, Feldstein VA, Goldstein RB, Filly RA. The "Empty Amnion": A sign of early pregnancy failure. J Ultrasound Med 1995;14:117-121.