Why Do Miscarriages Occur? Understanding the Science Behind Pregnancy Loss
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Miscarriage is one of the most common complications of pregnancy, yet it remains widely misunderstood. For many people, the experience is accompanied by feelings of guilt, confusion and unanswered questions. It is natural to wonder whether something you did caused the pregnancy to end.
In reality, most miscarriages are beyond anyone's control. Early pregnancy is an incredibly complex process that depends on the healthy development of an embryo, successful implantation into the uterus and the formation of a functioning placenta. If any of these processes are disrupted, the pregnancy may unfortunately end in miscarriage.
As a doctor who experienced six miscarriages before finally having my rainbow baby, I understand the search for answers. While every pregnancy is unique, understanding why miscarriages occur can help you make informed decisions and know when further investigations may be appropriate.
What Is a Miscarriage?
A miscarriage is the spontaneous loss of a pregnancy before 24 weeks' gestation in the UK, although around 80% occur during the first trimester.
It is estimated that around 1 in 4 pregnancies end in miscarriage, making pregnancy loss far more common than many people realise. Many occur before someone even knows they are pregnant.
How Does a Healthy Pregnancy Develop?
For a pregnancy to continue successfully, several complex biological processes must happen in perfect sequence.
First, a healthy egg and sperm must combine to form an embryo with the correct genetic material. The embryo then travels to the uterus, where it implants into the endometrium (the lining of the womb). Shortly afterwards, the placenta begins to develop, providing oxygen and nutrients to the growing baby.
Problems affecting the embryo, uterus, hormones, immune system or placenta can interrupt this process and result in pregnancy loss.
Chromosomal Abnormalities
The most common cause of first-trimester miscarriage is a chromosomal abnormality within the embryo.
Humans normally have 46 chromosomes arranged in 23 pairs. Occasionally, an egg or sperm contains too many or too few chromosomes, or an error occurs during the first few cell divisions after fertilisation. These abnormalities often prevent normal embryo development.
It is estimated that around 50–70% of first-trimester miscarriages are associated with chromosomal abnormalities.
Most chromosomal abnormalities occur by chance and are not inherited from either parent. However, a small number of recurrent miscarriages are caused by inherited chromosomal rearrangements, such as balanced translocations, which may be carried by an otherwise healthy parent. The risk of random chromosomal abnormalities also increases with maternal age because eggs become more susceptible to errors during cell division over time.
Egg Quality and Mitochondrial Function
Egg quality is influenced by many factors, including age, genetics and overall health.
Each egg contains hundreds of thousands of mitochondria, often described as the cell's "power stations". These mitochondria generate the energy required for chromosome separation, fertilisation and early embryo development.
As women age, mitochondrial function gradually declines, which may contribute to reduced fertility and an increased risk of chromosomal abnormalities.
For this reason, researchers have become increasingly interested in Coenzyme Q10 (CoQ10), a naturally occurring antioxidant involved in mitochondrial energy production. Several studies have investigated whether CoQ10 supplementation before conception may improve aspects of egg quality, particularly in women with reduced ovarian reserve or advancing maternal age.
Uterine Abnormalities
Sometimes the uterus itself may contribute to miscarriage.
Examples include:
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A uterine septum
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Large fibroids that distort the uterine cavity
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Significant scar tissue (Asherman's syndrome)
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Congenital abnormalities affecting the shape of the uterus
Not every uterine abnormality causes miscarriage, and many people with fibroids or anatomical differences have healthy pregnancies. However, when these conditions significantly interfere with implantation or placental development, treatment may be recommended.
Hormonal and Medical Conditions
Several medical conditions can increase miscarriage risk if they are not recognised or well controlled.
These include:
Thyroid disease
Both hypothyroidism and hyperthyroidism have been associated with pregnancy complications. Appropriate diagnosis and treatment before conception are important.
Diabetes
Poorly controlled diabetes around conception increases the risk of miscarriage and congenital abnormalities. Optimising blood glucose before pregnancy significantly improves outcomes.
Polycystic Ovary Syndrome (PCOS)
PCOS is associated with insulin resistance, ovulation problems and hormonal imbalance. Although many women with PCOS have healthy pregnancies, miscarriage rates appear to be modestly higher than in the general population.
Progesterone
Progesterone prepares and supports the uterine lining during early pregnancy. While low progesterone is often seen in pregnancies that are already failing, progesterone supplementation may be beneficial in selected people with early pregnancy bleeding and previous miscarriage, in line with current NICE guidance.
Antiphospholipid Syndrome
Antiphospholipid syndrome (APS) is one of the few well-established, treatable causes of recurrent miscarriage.
APS is an autoimmune condition in which antibodies increase the risk of abnormal blood clotting. This can affect placental development and blood flow to the pregnancy.
When diagnosed, treatment with low-dose aspirin and low molecular weight heparin has been shown to improve pregnancy outcomes.
Cervical Insufficiency
Unlike most first-trimester miscarriages, cervical insufficiency typically causes pregnancy loss during the second trimester.
The cervix begins to shorten and open prematurely, often without painful contractions. Previous cervical surgery, trauma or a history of second-trimester loss may increase the risk.
Treatment may include cervical-length monitoring, progesterone or a cervical stitch (cerclage).
Lifestyle Factors
Certain lifestyle factors are associated with an increased risk of miscarriage, including:
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Smoking
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Heavy alcohol consumption
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Recreational drug use
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Poorly controlled chronic medical conditions
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Severe obesity
However, it is important to remember that everyday activities such as walking, exercising, lifting shopping bags, working or sexual intercourse do not usually cause miscarriage in an otherwise healthy pregnancy.
Can Stress Cause a Miscarriage?
One of the most common concerns after pregnancy loss is whether stress was responsible.
Current evidence does not suggest that normal day-to-day emotional stress causes miscarriage. Feeling anxious, working long hours or experiencing an upsetting event is extremely unlikely to have caused an early pregnancy loss.
Why Do Some People Have Recurrent Miscarriages?
Recurrent miscarriage is usually defined as three or more consecutive pregnancy losses, although many specialists now begin investigations after two miscarriages depending on individual circumstances.
Investigations may include:
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Pelvic ultrasound
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Thyroid function tests
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Antiphospholipid antibody testing
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Diabetes screening
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Genetic testing in selected couples
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Analysis of pregnancy tissue after miscarriage
Despite extensive investigations, around half of recurrent miscarriages remain unexplained.
Encouragingly, many couples with unexplained recurrent miscarriage still go on to have a healthy pregnancy.
Can Miscarriage Be Prevented?
Unfortunately, many miscarriages caused by chromosomal abnormalities cannot be prevented.
However, optimising health before conception may reduce some avoidable risks. This includes:
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Stopping smoking
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Limiting alcohol
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Maintaining a healthy weight
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Managing thyroid disease or diabetes
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Taking folate before conception
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Correcting nutritional deficiencies
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Seeking specialist assessment after recurrent miscarriage
Nutrition Before Pregnancy
Preparing for pregnancy begins long before a positive pregnancy test. Egg maturation takes approximately three months, meaning nutritional status before conception may be important.
Folate contributes to maternal tissue growth during pregnancy and normal cell division. Vitamin D supports normal immune function and bone health, while Coenzyme Q10 plays a central role in mitochondrial energy production.
CoQ10 has received particular attention in fertility research because of its role in supporting cellular energy within egg cells. In a randomised controlled trial involving women with diminished ovarian reserve undergoing IVF or ICSI, supplementation with CoQ10 before treatment was associated with significantly higher fertilisation rates and a greater number of high-quality embryos, although improvements in live birth rates have not been consistently demonstrated. While no supplement has been proven to prevent miscarriage, ensuring adequate nutritional intake before conception is one of the few modifiable factors available.
When Should You Seek Medical Advice?
You should speak with your healthcare professional if you:
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Have experienced two or more miscarriages.
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Have had a second-trimester pregnancy loss.
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Have thyroid disease, diabetes or an autoimmune condition.
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Have irregular periods or fertility concerns.
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Experience severe pain, heavy bleeding or symptoms suggestive of an ectopic pregnancy.
Early assessment may help identify underlying causes and guide future management.
Final Thoughts
Miscarriage is common, but that does not make it any easier to experience. The majority of early pregnancy losses occur because the embryo was unable to develop normally, most often due to chromosomal abnormalities that are completely outside anyone's control.
Although miscarriage cannot always be prevented, understanding the possible causes, investigating recurrent pregnancy loss where appropriate and optimising health before conception can help provide the best possible foundation for a future pregnancy.
References
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Royal College of Obstetricians and Gynaecologists. The Investigation and Treatment of Couples with Recurrent First-trimester and Second-trimester Miscarriage (Green-top Guideline No. 17).
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National Institute for Health and Care Excellence. Ectopic pregnancy and miscarriage: diagnosis and initial management (NG126).
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American Society for Reproductive Medicine. Evaluation and treatment of recurrent pregnancy loss: a committee opinion.
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Bentov Y, et al. The role of Coenzyme Q10 in ovarian reserve and assisted reproduction. Fertility and Sterility.
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Xu Y, et al. Pretreatment with Coenzyme Q10 improves ovarian response and embryo quality in women with diminished ovarian reserve undergoing IVF-ICSI: a randomised controlled trial. Reproductive Biology and Endocrinology. 2018.
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NHS. Miscarriage.
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European Society of Human Reproduction and Embryology. ESHRE Guideline: Recurrent Pregnancy Loss.