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Placenta

Anomalies

Name:

Amniotic sheet

Description:

An amniotic sheet is due to a redundancy of the amnion and chorion. The location of an amniotic sheet within the uterus is highly variable. In one study the mean length and width of 79 amniotic sheets were 4.0 + 1.5 3 cm and 0.46 + 0.13 cm, respectively. When complications associated with a perpendicular or a parallel amniotic sheet were compared, breech presentation was significantly higher (50% vs. 3%; p < 0.001) with perpendicular amniotic sheets. The prevalence of vaginal bleeding was comparable between the two groups (perpendicular 36%; parallel, 43%). Since a control group was not provided in this study, it could not be determined if the recorded prevalence of bleeding was significant.

Diagnosis:

Amniotic sheets were identified in 79 of 17,533 (0.45%) ultrasound examinations performed between 12 and 28 weeks’ gestation. Amniotic sheets are progressively more difficult to visualize sonographically as gestational age advances.

Differential Diagnosis:

An amniotic sheet must be differentiated from an amniotic band. The amniotic band syndrome is due to rupture of the amnion with entanglement of various fetal parts. A diagnosis of an amniotic band is suggested by the presence of multiple structural defects due to a disruption of normal embryologic development (i.e., an encephalocele, a missing extremity, etc). Occasionally, the thin amniotic bands can be visualized adherent to the identified congenital malformation. On the other hand, an amniotic sheet is a thicker, straight fold of amnion and chorion that usually runs along a portion of the placenta. If the uterus was previously scarred by infection or instrumentation, a synechia may result. Once a patient with a uterine synechia becomes pregnant, the amnion enfolds around the synechia. Hence, a synechia is covered by amnion on both sides. A uterine synechia is sonographically similar to an amniotic sheet. Since a uterine synechia is within the uterus, it may affect fetal life.

Sonographic Features:

Amniotic sheets may occur either perpendicular or parallel to the placental surface. As the placenta increases in size, it stretches parallel amniotic sheets, increasing their length and pulling them closer to the main body of the placenta.

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References


Amniotic sheets: natural history and histology.
Stamm E, Waldstein G, Thickman D, McGregor J.
J Ultrasound Med;10:501-4.

The effect of amniotic sheet orientation on subsequent maternal and fetal complications.
Lazebnik N, Hill LM, Many A, Martin JG.
Ultrasound Obstet Gynecol;8:267- 71.

Amniotic sheets: Ultrasonography and pathologic correlation.
Nelson LH, King M, Lantz P.
J Ultrasound Med;15:543-45.