What happens normally in the menstrual cycle?

The menstrual cycle is regulated by a number of hormones. Some of these hormones (FSH and LH) are produced by the master gland in the brain called the pituitary and in response to these hormones the ovaries produce oestrogen and progesterone. The production of these hormones each month leads to the build-up of the lining of the womb (called endometrium) and the release of an egg from the ovary (called ovulation). Pregnancy occurs if the egg is fertilised by a sperm. If the egg is not fertilised, then the lining of the womb is shed, and bleeding occurs (called a period).  Then the process begins again for the following month.

There is usually a fine balance in the production of these hormones in each menstrual cycle, which results in a regular monthly period occurring approximately every 28 days.

What happens to the menstrual cycle in women with PCOS?

Hormonal disturbances are common in women with PCOS, with high levels of hormones called androgens. This results in the ovary developing lots of small follicles (sacks of fluid) which contain eggs at an early stage of their development. These follicles fail to develop in a way that would allow egg release.

Due to these hormonal disturbances and failure of egg release, women often have less frequent periods, no periods at all, although they can also have more frequent and heavy bleeding.  This can happen in girls/women soon after starting their periods, if they start at all, but can also occur if they put on weight.

What are the consequences of not having regular periods?

If egg release does not occur on a regular basis, and therefore no regular bleed, this can cause the lining of the womb to become thicker than usual. This can predispose to endometrial hyperplasia (an overgrowth of the lining of the womb), which is thought to put women at higher risk of endometrial cancer later on in life (average age of for this cancer to be diagnosed is 60). To minimise this risk, a woman should have at least 4 menstrual bleeds per year so that the lining of the womb remains healthy.

Another consequence of having infrequent periods is that this indicates reduced opportunities to get pregnant, as the eggs are not matured and released every month in the normal way. Please speak to your doctor if you have concerns about becoming pregnant now or in the future.

What will happen in clinic?

Your doctor will ask about how many periods you have a year and what they are like (are they painful? do you experience heavy or light bleeding? etc.). Your doctor may measure hormones in your blood and organize an ultrasound scan (ideally an internal scan, if appropriate) to both look for follicles in your ovaries and the lining of your womb.

If your periods are less than 4 times a year or there is longer than 3 months in between periods, certain medications such as contraceptive pills and medication containing progesterone may be used to keep your endometrium lining healthy and make your periods more predictable or less heavy.

What treatments are available to help with absent or irregular periods in women with PCOS?

Weight loss:

Weight loss of 5-10% can help women with PCOS to experience more regular periods and enhance natural fertility which can be reassuring to many women with PCOS who wish to avoid the use of medical treatment options.

(see leaflet on PCOS and body weight)

Combined Oral Contraceptive Pill:

This contains oestrogen and progesterone and is an option for women with PCOS with irregular periods either to protect endometrial lining or just make their periods more predictable or less heavy.  It also has the advantage of preventing unwanted pregnancies and it can help with problems of excess hair.

What are the side effects?

The main risk of taking the combined oral contraceptive pill is the risk of blood clots in your leg veins or lungs.  This risk is higher in those who are older, more overweight, smoke, have diabetes or hypertension and have a family history of clots. Information leaflets on commonly prescribed oral contraceptive formulations are available from clinic.  The risks of clots for each preparation are outlined in these documents.

Women with particular subtypes of migraine may not be able to have it. Common side effects that you may experience can include nausea, bloating, breast tenderness, headaches, mood swings, and occasionally bleeding in between your periods. These can be discussed with your healthcare provider when considering treatment and these will be tailored to your own needs.

Progesterone Preparations:

The hormone progesterone is naturally produced by the ovary after egg release and prepares the lining of the womb for pregnancy, when this does not happen and progesterone levels drop, a period occurs. In addition, the hormone progesterone keeps the lining of the womb thin and healthy. They may be preferred in women at higher risk of developing the side effects of the combined pill which are mentioned above. There are various options for using progesterone alone when periods are irregular:

  • Short courses of progesterone tablets, after which you have a bleed, which maintain the health of your endometrium, although this method will not prevent unwanted pregnancy.
  • Progesterone only contraceptive pill which would keep the endometrium healthy and prevent unwanted pregnancies.
  • Intra-uterine system delivering progesterone, for example, Mirena coil, which releases a small amount of progesterone direct to the endometrium and keeps the endometrium healthy and prevent unwanted pregnancies.
  • Depot injection or implant also introduce progesterone into the body and therefore keep the endometrium healthy and prevent unwanted pregnancies.

What are the side effects?

Some side effects that you may experience while on the progesterone only pill include skin changes, spots, breast tenderness and enlargement, nausea, headaches, and occasionally changes in your mood.

It is important to discuss these side effects with your healthcare provider when you are selecting the pill that suits you and also if you develop any of these side effects while on treatment.


Metformin is a medication, which is commonly used to treat diabetes; however, it can also have a role in making periods more regular in women with PCOS. Metformin also improves the way the body responds to insulin and can help maintain a healthy body weight, which is also important in the overall treatment of PCOS.

What are the side effects?

Metformin is usually taken once or twice daily and the main side effects to be aware of is that it can occasionally cause nausea, vomiting, and loose stools. Your doctor may be able to minimise these effects if the medication is started at a low dose and gradually built up. It should be taken with your breakfast, or with breakfast and evening meal if taken twice daily. There are also slow release formulations which may be associated with fewer gastrointestinal side effects.. If you are sick and unwell, your doctor may temporarily pause this medication while you recover from your illness.

It is also used in pregnant women to treat diabetes in pregnancy and does not necessarily need to be stopped if you fall pregnant whilst taking metformin.

This information leaflet has been co-authored by:
Dr. Michael O’Reilly (Consultant Endocrinologist, Beaumont Hospital)
Ms. Maureen Busby (CEO and Founder, PCOS Vitality patient support group https://www.pcosvitality.com/what-is-pcos)

Useful links

© 2023 Polycystic Ovary Syndrome PCOS Global