This is called a decidual cast. It occurs when the lining of a

Decidua - Wikipedia

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Decidual Cast: Structure, Symptoms, Causes

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Decidual Casts & Birth Control

Decidual cast from a woman - Stock Image - C058/5454 - Science Photo Library

I think I passed a decidual cast (TMI PICTURE)

Woman Goes Viral for Detailing Rare Experience of Decidual Cast, Shedding Uterus Lining All At Once