Ovulation is primarily caused by preovulatory surge of

Estrogen 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; page no:-406