The basic problem appears to be in the ovary. The human ovary has a fixed number of eggs within it, which is determined well before birth. From the time of puberty, in each menstrual cycle, under the influence of hormones, a group of these eggs begin to grow in little drops of fluid (called follicles). When this process is well regulated, one follicle becomes dominant and releases the egg in mid-cycle (ovulation), while the remaining follicles undergo degeneration and disappear. This process is repeated every month with regular ovulation and menstruation if pregnancy is not achieved.

In women with PCOS, this process becomes disorganised. Due to an imbalance in the sequence of the hormones, the follicles containing the eggs within the ovary fail to grow beyond a particular stage. These then cannot progress forward towards ovulation, and are also unable to disappear as they do normally. They appear on ultrasound scan as small (5-10mm) fluid filled follicles or ‘cysts’ within the ovary and distributed along the periphery of the ovary.

These follicles then produce a higher amount of the hormone testosterone within the ovary, which then leads to a vicious cycle preventing further follicular growth or ovulation. Each new batch of these follicles which begin to grow are similarly affected.