The earliest morphological evidence of ovulation on endometrial biopsy is :

Endometrial biopsy Consists of curretting small pieces of endometrium from the uterus with small endometrial biopsy curette, preferably 1-2 days prior to menstruation Material removed is fixed immediately in formalin saline Secretory changes (basal vacuolation) proves that cycle has been ovulatory Ref: Shaw's Textbook of Gynaecology, 15th edition, page no: 215