Understanding the Causes of Infertility-image

Understanding the Causes of Infertility

Medically Reviewed by Hertility on March 28, 2024

For many, the journey to parenthood can be a challenging process, both physically and emotionally. We’re here to help you understand the different causes of infertility, and the options available for those who need support.

Quick facts:

  • Infertility is when you’re unable to get pregnant after a year of trying. 
  • Infertility is really common, with 1 in 7 heterosexual couples struggling with it. 
  • Underlying health conditions, structural issues, infections and hormone issues can all be causes of infertility. 
  • Infertility can be treated depending on the cause.

What is infertility?

Infertility is defined as not being able to conceive after one year (or longer) of trying. This could be despite having regular unprotected sex with a partner, or trying using methods like artificial insemination (IUI). 

There are 2 types of infertility:

  • Primary infertility: if it’s your first time trying to get pregnant and you’ve not conceived and carried a pregnancy to term before. 
  • Secondary infertility: if you have had one or more pregnancies in the past but you’re struggling to conceive again. 

In the UK, as many as 1 in 7 heterosexual couples experience infertility, yet the causes are sometimes preventative, or treatable. 

Causes of infertility 

There can be many different reasons why you might struggle to conceive. This can include structural fertility issues, ovulation problems, underlying health conditions and hormonal imbalances. Let’s take a look at each.

Ovulation issues 

Anovulation, also known as the inability to ovulate, is the most common cause of infertility. Ovulation is when a mature egg is released from one of your ovaries, each month, during your menstrual cycle. The egg travels into the Fallopian tube, where it prepares to be fertilised by a sperm, before then travelling down the tube to the uterus. 

When trying to conceive, ovulation is a crucial event, with the 5 days before ovulation and ovulation day itself often referred to as your ‘fertile window’.This is the time of the month when you’ll be most likely to get pregnant. 

Research suggests that as many as 25% of infertility cases are caused by anovulation. Anovulation can be caused by:

  • Gynaecological or ovarian complications: such as Polycystic Ovary Syndrome (PCOS) or Primary Ovarian Insufficiency (POI). 
  • Age: As you age your ovarian reserve declines, causing a hormonal imbalance which can result in irregular periods.
  • Endocrine disorders: Certain conditions that affect your reproductive hormones in your body such as thyroid disease or defects in the hypothalamus, can also affect the hormones that trigger ovulation.
  • Lifestyle and environmental factors: such as smoking, an unhealthy diet, consuming excess alcohol and/or drugs and more.
  • Problems in the menstrual cycle.

Another possible explanation is a problem with the egg maturation process. This means that an “immature” egg may be released from your ovaries when it is not quite ready and unable to fertilise.

Underlying health conditions

Polycystic ovary syndrome (PCOS)

PCOS is the most common underlying condition affecting fertility. PCOS affects as many as 1 in 10 people with ovaries. 

People with PCOS produce higher levels of androgen hormones, like testosterone. This can disrupt your menstrual cycle, ovulation, and balance of cycling hormones—like oestrogen, follicle-stimulating hormone and luteinising hormone (LH).

As PCOS is still under-researched and misunderstood, many people are left undiagnosed and are unaware they have it until they are actively trying to conceive. This is why it’s a good idea to check in on your hormones before you start your conception journey. 

Primary ovarian insufficiency (POI) 

POI is when the ovaries stop working properly before the age of 40. POI is far less common than PCOS, affecting only 1 in 100 women younger than 40.

Your ovaries produce oestrogen and progesterone—two cycling hormones responsible for the regulation of your menstrual cycle. When their function is disrupted, ovulation can also become disrupted or stop completely. 

People with POI also have a lower ovarian reserve, meaning fewer eggs for their age. Without a sufficient amount of eggs and the correct level of hormones needed to regulate your cycle, the possibility of conceiving is reduced considerably. According to the National Infertility Association, POI can be caused by genetics, autoimmune factors, induced by chemo or radiotherapy, or it can have an unknown cause.

Uterine fibroids

Uterine fibroids are noncancerous tumours that affect as many as 25% of people with a uterus. They can cause symptoms such as heavy periods, intermenstrual bleeding, and pelvic pain.

In some cases, depending on the location in which they grow, and whether they affect the shape of the uterus or cervix, fibroids can also cause infertility. 

This is because they can block the fallopian tubes, stopping eggs from meeting sperm, or reaching the uterus for implantation. They can also get in the way of implantation if they are near the inner lining of the womb. 

Despite being less common than other causes of infertility, around 5-10% of infertile women and those assigned female-at-birth are found to have uterine fibroids, with their instance being much more common in Black women. 


Endometriosis is a common reproductive health condition that affects around 1 in 10 women and people assigned female-at-birth. With endometriosis, tissue similar to the tissue that makes up the lining of the womb starts to grow in other places, such as the ovaries and the fallopian tubes, causing lesions and scarring.  

This can cause several life-altering symptoms, such as painful and heavy periods, pain during or after intercourse and pelvic pain. It is also another common cause of infertility, with research suggesting it is present in around 20-50% of infertility cases.

The exact link between endometriosis and infertility is unknown. However, some theories suggest that lesions and scarring can cause structural problems with the reproductive organs and chemical changes in the lining of the uterus.

Structural infertility 

Structural infertility is a problem with the anatomical structure of the reproductive organs. This can include blockages, structural damage or abnormal growth in the fallopian tubes, uterus or ovaries. 

Structural issues with the ovaries or fallopian tubes can prevent eggs from being released from the ovaries or stop them from moving through the fallopian tubes to reach a sperm for fertilisation.  

Additionally, if the structure of your uterus is abnormal or damaged, it may prevent eggs from implanting into the endometrium (uterus lining). This is needed to create a healthy embryo and pregnancy. 

Structural infertility problems can also be caused by scarring from surgery, infections, injuries, or endometriosis. 

Also, the growth of noncancerous tissues such as uterine polyps on the lining of the uterus, can cause blockages. Polyps occur when additional tissue grows on your uterus. However, sometimes tissue grows elsewhere in your reproductive system potentially blocking your fallopian tubes and preventing pregnancy. 

Implantation failure

Other possible explanations of implantation failure are:

  • Defects in the embryo are either genetic or simply embryonic
  • Endometrium Defects: A defect in the wall of your uterus
  • Endometriosis
  • Hormone resistance, in particular, progesterone.
  • Scar tissue in the endometrium.

Infections and auto-immune disorders 

Untreated sexually transmitted infections (STIs) may have serious consequences for your fertility. STIs like chlamydia or gonorrhoea can cause scarring and blocking of your Fallopian tubes. 

Additionally, if syphilis is left to develop, it can cause stillbirth. There are also other forms of infections of the cervix with human papillomavirus (HPV) that could cause infertility.

It’s important to regularly get checked for STIs if you’re having sex with new partners. 

Dealing with infertility

Experiencing infertility can be extremely challenging and can understandably take its toll on your mental wellbeing. At Hertility, we can help you if you’re struggling with infertility. 

With our at-home Hormone & Fertility tests, we can check your hormone levels and screen you for up to 18 health conditions that may be impacting your fertility. We also have a team of private gynaecologists who can prescribe you a personalised care plan. 

If you are struggling mentally, we can also support you with the emotional aspects of infertility with our fertility counsellors


  1. https://www.nhs.uk/conditions/infertility/
  2. https://www.nhs.uk/conditions/infertility/causes/
  3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5936612/
Meeladah Ghani BSc (hons), MSc

Meeladah Ghani BSc (hons), MSc

Meeladah is a Ph.D. Research Student at University College London (UCL), studying her Doctoral degree with the EGA Institute of Women’s Health. Her research is focused on investigating Translational Ovarian Physiology and Pathophysiology. More specifically, the various implications of PCOS. Meeladah also has a Bachelor and Master in Experimental Pharmacology & Therapeutics, during which she was awarded the Royal Society of Biology award for her research study, which was awarded in the Houses of Parliament, on novel medications used to treat PCOS.

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