Ovum is released due to

Ovulation-the discharge of a secondary oocyte from the Graafian follicle; in an adult woman, this normally occurs at intervals of about28 days and alternates between the two ovaries. As a rule, only one secondary oocyte is produced, but occasionally ovulation produces two or more; if more than one subsequently become feilized, the result may be multiple bihs, such as twinsEstrogen levels peak towards the end of the follicular phase. This causes a surge in levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This lasts from 24 to 36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary the oviduct.Ref: Ganong&;s review of medical physiology; 24th edition