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My water broke on the evening of the due date.
Literal
Due-date [possessive-の] evening [at-に] water-broke.
A terse, matter-of-fact medical statement. 破水 (water breaking) is the specific obstetric term. 予定日 (due date/scheduled date) is used across contexts but here specifically means the expected delivery date. The simplicity of the sentence reflects the direct, diary-like style of personal health accounts.