Why Is My Period Late? 15 Reasons for A Missed Period

Table of Contents

• 15 reasons for a late period
• When to see your doctor

15 reasons for a late period

Here are 15 reasons to explain why your period can be late:

1. Pregnancy

If you’ve been sexually active within the last month and your period is late, you’re going to worry. Pregnancy is the most common cause of a late or missed period. Early pregnancy symptoms such as cramps, bloating, nausea, fatigue, breast tenderness and spotting when you wipe (or in your underwear), are very similar to the symptoms you get towards the end of your cycle. This makes it difficult to tell if your cycle is off by a few days, or if you're pregnant.

If your period is a week late and you’ve had sex within the last few weeks, the most sensible course of action is to do a pregnancy test. Even if you’re using contraception, you can’t rule out pregnancy altogether, as no contraception is 100 per cent effective.

Most pregnancy tests can be taken from the first day of a late or missed period. If you’re not sure of the exact date your next period is due, the NHS recommends doing a pregnancy test at least 21 days after your last had unprotected sex.

Pregnancy tests are widely available in pharmacies and supermarkets. You can get one for free from your GP surgery, sexual health clinic, or a Brooks Centre if you’re under 25. 

2. Stress

Stress can be one of the main reasons for a late period. As stress levels rise, a woman’s reproductive system and hormone balance can be affected. Research shows that for some women, stress can play a role in causing your period to be delayed.

When stress levels reduce, your menstrual cycle will usually return to its normal pattern, without any long-lasting effects. Try not to worry about a late period that may be caused by stress. If you are feeling stressed, take time to relax, exercise, or try breathing exercises. You should contact your doctor for support, if your stress is overwhelming or hard-to-manage.

3. Hormones

Hormone imbalances involving prolactin or thyroid hormones, can cause a woman’s period to be late. These imbalances can be easily identified with a blood test.

If you have a hormonal imbalance, your doctor will investigate the reasons for it. Hormonal medication can help your periods to return to their normal cycle.

4. Being overweight

Being overweight or suddenly putting on a significant amount of weight, can affect your menstrual cycle by causing excess oestrogen production. This can lead to a late or missed period.

If you think that your weight is causing period irregularities, you should speak to your doctor. Obesity and missed periods can sometimes indicate a medical condition, such as polycystic ovary syndrome (PCOS), so it is important to get it checked out. Alternatively, your doctor will be able to provide you with healthcare advice and signpost you to services which can support you to lose weight. Weight loss medication is also an option where lifestyle measure have failed. 

5. Being underweight

On the flipside, significant weight loss and low body weight can cause a woman to miss her period. Being underweight or having too little body fat compared to your body mass, known as a low body-fat ratio, can reduce your oestrogen reproductive hormone levels so that ovulation and menstruation do not occur.

You should see your doctor or a dietician if you have excessive weight loss or you are underweight. They can help you to get the right amounts of nutrients your body needs to function correctly and resume your menstrual cycle.

6. Thyroid problems

Thyroid problems can cause abnormal menstrual changes. Your thyroid is a gland responsible for your body's metabolism.

If your thyroid is overactive, known as hyperthyroidism, your periods can become lighter, less frequent or absent. Additional symptoms of hyperthyroidism include weight loss, rapid heartbeat, increased sweating and trouble sleeping.

If you have an underactive thyroid, also known as hypothyroidism, your periods may be less frequent, but heavier. Other symptoms of hypothyroidism include weight gain, fatigue, dry skin and hair loss.

A blood test can help your doctor determine if you have a thyroid problem. Medication is available to help you

7. Perimenopause

Menopause usually occurs in women between the ages of 45 and 55. It is officially recognised when a woman hasn’t had a period for at least 12 months. After menopause, you won’t have a period again.

Perimenopause, also known as the menopause transition, begins several years before menopause. It is a time when your body starts transitioning to menopause. During this time, your ovaries gradually begin to produce less oestrogen. This can cause changes in your menstrual cycle. Your periods may come more or less frequently, be shorter or longer, or be lighter or heavier.

Other symptoms of perimenopause include mood swings, hot flushes and night sweats, brain fog, sleeping difficulties, vaginal dryness, and vaginal burning.

The average ager for perimenopause is between 40-45. However, some women go through it as early as their 30's.

You should see your doctor if you have symptoms that are concerning, or you haven’t had a period for 3 months (especially if you are under 45 years). They can do a blood test to check your hormone levels and help pinpoint the cause.

8. The contraceptive pill

If you have recently started, changed, or stopped taking hormonal contraception, there’s a good chance your periods will be affected. Your body needs to adapt to the changes in hormone levels caused by taking the contraceptive pill. This may result in your period being less frequent or lighter. Some women stop bleeding altogether when taking the pill. 

The contraceptive pill and many other hormonal birth control options work by preventing ovulation.  They do this by altering the levels of natural hormones your body makes. Many also thin the lining of your womb to a point where a period is not necessary. Usually, this isn’t harmful, but you should speak to a doctor with any concerns about your birth control method.

You should also bear in mind that no contraceptive pill offers 100% pregnancy prevention. So, if your period it is late, you may wish to take a test.

9. Polycystic Ovary Syndrome (PCOS)

Polycystic ovary syndrome (PCOS) is a common medical condition affecting hormone levels. It can result in persistently late or irregular periods, and in some women, it can stop periods altogether.

PCOS is caused by a hormonal imbalance in women. Polycystic ovaries have many underdeveloped sacs in which eggs develop. Often these sacs are unable to release an egg, resulting in no ovulation.

If you have PCOS you may experience other symptoms as well as changes to your periods. These include excessive hair growth or thinning hair, weight gain, acne and difficulty becoming pregnant because of irregular or missed periods. Some women with PCOS don’t get any symptoms.

You should see your doctor if you think you have PCOS symptoms. They may be able to offer treatment to relieve them. Often lifestyle modifications are recommended for women with PCOS such as healthy eating and weight loss.

10. Changes in Diet

Changes in your diet can cause your period to be late or irregular. Research shows that hormone production can be affected by diet, which can affect your menstrual cycle.

If you limit your calorie intake and range of foods, you may prevent your body from getting enough nutrients to meet your hormonal needs. Depending on the extent of your diet, it is possible for your body to enter starvation mode and use the nutrients for other vital areas, such as your brain and heart health. This can result in missed, irregular or late periods. Women with eating disorders such as anorexia nervosa and bulimia, often have irregular or missed periods.

A diet lacking minerals and vitamins such as iron, calcium, magnesium and zinc, folate, or vitamins B, C and D, are associated with menstrual cycle changes.

11. Exercise

Excessive exercise and intense physical activity can cause hormonal imbalance. This is common amongst women training for intense sporting events such as marathons. In fact, many professional athletes experience irregular period patterns.  

Moderate exercise, such as going to the gym, swimming, or cycling a few times a week, is very important for your general health and will not usually impact periods. But putting your body under intense physical stress may affect your adrenal, thyroid and pituitary glands, causing a hormonal imbalance. This, in turn, impacts ovulation and can change your menstrual cycle, resulting in late, irregular, lighter or missed periods.

If you are planning on undertaking a strenuous exercise regime, optimising your diet with nutritious and energy-boosting foods will help to support a healthy period cycle. If your periods have become irregular or stopped altogether due to a significant increase in exercise, you should talk to your doctor.

12. Breastfeeding

If you’re breastfeeding, your periods may be irregular or absent. They might not return at all until you stop breastfeeding. This is because the hormones responsible for milk production also suppress ovulation.

You should be aware that you are still fertile whilst breastfeeding, even if you are not getting periods. Many women think they cannot fall pregnant whilst breastfeeding. This is not true. You need to use contraception if you do not wish to fall pregnant.

13. Travelling or disrupting your sleep cycle

If you’re jet-lagged from travelling, a shift worker changing schedule, or you suffer from disrupted sleep patterns, you may find that your period turns up earlier or later than expected.

Your menstrual cycle is closely associated with your circadian rhythm. Circadian rhythms are 24-hour cycles that are part of the body's internal clock. They help to regulate your hormone production and can affect ovulation and your periods as a consequence.  

The good news is that once you settle into a new regular wake-sleep pattern, your hormone levels should readjust, and your period should return to normal.

14. Long-term medical conditions

There are some long-term medical conditions that can also cause late or irregular periods. Women with diabetes may have an increased risk of experiencing irregular or unpredictable menstrual cycles. Remember, these are long term conditions and so period changes happen gradually over a long period of time, as opposed to a one off.

If you have ongoing period changes you should see your doctor.

It’s not only long-term illness that can throw your period out of sync. The added pysical toll that an acute short-term illness has on your body, can delay your period. This is because your body prioritises its resources to help you get better.

15. Covid-19 vaccine

Over 30,000 women have reported to the MHRA Yellow Card scheme that they have had changes to their menstrual cycle after having the COVID-19 vaccine. Changes include frequency, duration, regularity and volume of menstruation. It is very difficult to know if these changes are a direct result of the vaccine itself, or due to the wider effects of the pandemic. In this BMJ article, it states that most of these changes are temporary and periods return to normal after a single cycle.

When to see your doctor

There are many things that can affect your period and cause it to be late. In fact, it might even be normal for your period to be late. However, what is considered to be “normal”, will depend on the pattern of your average cycle. 

If you miss your period, and any of the following apply, you should see your doctor:

• If you have taken a pregnancy test that has given a positive result, you should let your doctor know that you are pregnant. They can book your antenatal follow up and give you all the information and support you need for a healthy pregnancy.
• You are experiencing overwhelming and hard-to-manage stress – your doctor can advise on ways to reduce your stress such as exercise and healthy habits.
• You think you may have a hormonal imbalance – your doctor will be able to do a blood test to check.
• You are underweight, overweight, or obese – your doctor can offer nutritional advice, medication to help lose weight, or refer you on to a dietician.
• You have an overactive or underactive thyroid - your doctor can arrange a blood test to determine if you have a thyroid problem. Medication is available to help manage this.
• You think you are in early perimenopause. – if you are under 45 years and haven’t had a period for 3 months. Your doctor can do a blood test to check your hormone levels.
• You are concerned about your contraceptive pill – your doctor may be able to recommend alternative birth control for you to try.
• You have PCOS symptoms and think you have the condition – your doctor will offer advice on lifestyle changes such as healthy eating and weight loss to alleviate your symptoms. Medication is available to help manage PCOS.
• You have an eating disorder such as anorexia nervosa or bulimia – your doctor can arrange for support such as a dietician and counselling to help you with your eating habits.
• You are planning or undertaking excessive exercise, such as training for a marathon – your doctor can give you advice to optimise your diet with nutritive and energy-giving foods.
• You suffer from disrupted sleep or insomnia – your doctor can advise or refer you to a specialist to help you change your sleep habits and address any issues that may be associated with your poor sleep or insomnia.
• You are taking period delay tablets such as norethisterone, and your period has not come within a few days of stopping – Pregnancy is possible, however, if you have a negative pregnancy test, you should have this looked into.

Whilst all of our content is written and reviewed by healthcare professionals, it is not intended to be substituted for or used as medical advice. If you have any questions or concerns about your health, please speak to your doctor.