Every month any one of a woman’s ovaries produces an egg (which is inside a follicle) which begins to grow after onset of menses (usually from day 8-9). This egg reaches maturity at about 18-22 mm size (about day 14) and then the follicle ruptures to release the egg which may now be fertilised by sperm. This process of egg rupture is known as ovulation. It usually happens on day 14-15 of a 28-30 day menstrual cycle.