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The specialized ENDOMETRIUM of pregnancy. Estrogen (ESTROGENS), PROGESTERONE, and other stimuli from the implanting BLASTOCYST transform the secretory endometrium into decidua. Changes include the enlarging of the endometrial STROMAL CELLS into round decidual cells. Each becomes surrounded by a membrane, which forms a scaffolding for TROPHOBLAST attachment. (From Cunningham et al, Williams Obstetrics, 20th ed, p87)