A womens ovary has 6 million oocytes at puberty. At every menstrual cycle a group of antral follicles, 10-15 in number start growing. An antral follicle consists of an oocyte (female egg) surrounded by granulosa and theca cells (two types of cells)- which nourishes the oocyte and secrete female hormone called estrogen. As estrogen collects around the oocyte the antral follicles look like small fluid filled cysts.
They grow at a rate of 2 mm per day, by the 5th or 6th day of menstrual cycle, majority of these antral follicles undergo artesia and only one grows. This growing follicle can be seen by Transvaginal ultrasound daily- This is called follicular study. The study starts around 8th to 10th day of menstrual cycle and continues till the follicle ruptures. It usually ruptures after it achieves 20 mm size.
In regularly menstruating women, follicle ruptures 14 days before expected menses; that is follicle will rupture on 16th day in a women who has 30 days cycle. With rupture of the follicle, oocyte with few granulosa cells falls out of the ovary and is picked up by the fallopian tube.