The cause of polycystic ovary syndrome (PCOS) is not fully understood, but genetics may be a factor. If you have PCOS, your sisters and daughters have a 50% chance of developing PCOS.¹

The first signs of PCOS are usually after a girl's menstrual cycle begins (menarche). A teen with menstrual periods over 45 days apart may need to be seen by a doctor to make sure she doesn't have PCOS. (Normally, the first periods and ovulation are hard to predict. They become regular within the first 2 years after menarche. For more information, see the topic Normal Menstrual Cycle.) In some women, PCOS starts after a big weight gain.²³

PCOS problems are caused by hormone changes. One hormone change triggers another, which changes another. This makes a "vicious circle" of out-of-balance hormones in your endocrine system, including:

• Ovary hormones. When the hormones that trigger ovulation are not at the right levels, the ovary does not release an egg every month. In some women, cysts form on the ovaries. These cysts make androgen.
• High androgen levels. High androgen in a woman causes male-type hair and acne problems and can stop ovulation.
• High insulin and blood sugar levels. About half of women with PCOS have a problem with how the body uses insulin, called insulin resistance. When the body doesn't use insulin well, blood sugar builds to high levels. If not treated, this can lead to diabetes.