Watery period? Here are the most common reasons why

If you’ve noticed that your period has become thinner and more watery in consistency lately, you might be wondering why.

Periods — and the entire menstrual cycle — are tightly controlled by the (roughly) monthly pattern of hormonal changes that makes pregnancy possible. When something happens to disrupt this balance, it can cause your bleed to change in consistency, colour, length, and more.

In this article, we’ll explain why your period blood might be watery, and give you some guidance about when to seek medical advice from a doctor.

What makes a period watery?

During the menstrual cycle, rising oestrogen levels prompt the lining of the uterus to thicken. After an egg is released from the ovary — around the middle of the menstrual cycle — rising progesterone levels prepare the thickened uterine lining for the implantation of a fertilised egg. If no egg is fertilised, progesterone levels fall, and the uterine lining is shed.¹

Since oestrogen is the main hormone prompting the uterine lining to thicken, low oestrogen levels in the body can lead to a thinner uterine lining. This means there is less to shed as a period. Progesterone also affects the thickness of the uterine lining, by opposing the effect of oestrogen.² When progesterone hormones are given — for example, as part of contraception — this can prevent the uterine lining becoming too thick, mimicking the effect of low oestrogen.

Why is my period watery?

The timing and length of periods, and the consistency and colour of period blood, can all be affected by hormonal changes.

If you have noticed that your period blood seems lighter or more watery than usual, here are some possible reasons why:

You have recently started hormonal contraception

The progesterone in hormonal contraception can alter the process by which the uterine lining builds up, and can affect your period bleeding. The progesterone IUS, for example, prevents pregnancy by making the lining of the uterus thin and unsuitable for a fertilised egg to implant.³

Although most other forms of hormonal contraception (e.g. the combined or progesterone-only pills, the implant, and the injection) primarily work by stopping ovulation,^4,5,6 the progesterone contained in them can also act to thin the uterine lining, and thereby make periods lighter. This can sometimes mean that periods are more watery.

You are approaching the menopause

The menopause is defined as 12 consecutive months without having a period. Before this, during what’s known as the “perimenopause”, hormone levels fluctuate unpredictably, and this can affect your periods. This can also cause a variety of symptoms such as anxiety, hot flushes and irregular periods.

Although some people find that their periods become heavier during the perimenopause, others report that their periods become lighter and more watery.

Oestrogen levels naturally decrease during perimenopause and after menopause, but other medical conditions can cause changes in oestrogen levels, too

You have low levels of oestrogen

Low levels of oestrogen in the body can mean that period blood is lighter, thinner or more watery.

Oestrogen levels naturally decrease during perimenopause and after menopause, but other medical conditions can cause changes in oestrogen levels, too, by affecting the HPO axis—the communication channel between the brain and ovaries that controls how much oestrogen is produced. If oestrogen levels are low, the uterine lining may be thinner, leading to a more watery period.

Very intensive exercise or restrictive diets — for example, due to eating disorders — can affect the HPO axis and lead to lower oestrogen levels.⁷ Sometimes this means that periods will stop entirely, but lower oestrogen levels can also result in lighter, more watery periods.

If the pituitary gland is not functioning as normal (what’s known as hypopituitarism) this can also cause lower oestrogen levels. Hypopituitarism can occur for a number of reasons, most often due to a benign (non-cancerous) pituitary gland tumour.⁸

You have more vaginal discharge than usual

An increase in vaginal discharge might make your periods seem more watery than they are. The amount and consistency of vaginal discharge naturally varies during the menstrual cycle and between different people, but a consistent change in your discharge might indicate something is wrong. Bacterial vaginosis, a type of vaginal infection, is known for causing very watery, fishy-smelling discharge.⁹

If you develop bacterial vaginosis around the time of your period, this may make your period blood seem more watery. Some sexually transmitted infections (STIs) can also cause watery discharge.

You are pregnant and experiencing implantation bleeding

When a fertilised egg implants in the uterus, ready to develop into a pregnancy, there can sometimes be a small amount of disruption to the uterine lining. This can lead to a small amount of light bleeding,¹⁰ which may have a watery appearance. This bleeding can sometimes be mistaken for a period.

When should I see a doctor?

Although periods can vary from one month to the next, there are some changes that you should speak about with a doctor. These include:

If you notice vaginal discharge with an unpleasant smell

A change in the smell of your vaginal discharge may indicate bacterial vaginosis or an STI. A doctor will be able to advise whether further testing is required based on your symptoms, and can prescribe the appropriate medications. Many STIs don’t have symptoms, so it’s worthwhile getting tested regularly even if you’ve not noticed any changes.

If you are concerned you may have a medical condition affecting your oestrogen levels

As explained above, some medical conditions like hypopituitarism can cause hormonal changes which might lead to watery periods. If you have any other symptoms such as tiredness, headaches or visual changes, speak to a doctor. A doctor can also support you if you are concerned you may have an eating disorder or if you think you may be over-exercising.

If you are experiencing troublesome perimenopausal symptoms, your doctor can suggest strategies to manage these

If you are pregnant and have noticed spotting or bleeding

Although small amounts of spotting in pregnancy is relatively common, bleeding whilst pregnant could indicate something more serious like a miscarriage or an ectopic pregnancy (when the pregnancy develops outside of the uterus).¹¹

If you have gone through the menopause and begin bleeding again

This should always be investigated further by a doctor. Vaginal bleeding after the menopause can be a sign of conditions such as uterine polyps (growths, usually non-cancerous), over-thickening of the womb lining (endometrial hyperplasia), or in rare cases, uterine/ovarian cancer.¹²

If you think you might be approaching the menopause and are struggling with symptoms

If you are experiencing troublesome perimenopausal symptoms (such as hot flushes, very heavy periods, or mood swings), your doctor can suggest strategies to manage these. For example, certain types of hormonal contraception can be used to manage heavy periods, and hormone replacement therapy can help some people experiencing mood swings.¹³

Featured image is an illustration of period pads lined up in a geometric pattern, against a salmon pink background

Page last updated June 2023
Next update due 2026