Causes of Miscarriage

Miscarriages are devastating, but what can also be difficult, particularly if you’ve suffered several losses, is not knowing what caused your loss and searching for answers.


There are a number of possible reasons that a miscarriage may occur, and often it’s not until extensive testing is done that you find out why. However, it’s also important to know that in around 50% of cases, a medical explanation will not be found. For couples in this category, most medical professionals will say that the chance of a healthy pregnancy is no different to the statistical chance of pregnancy of any other couple (taking into account the woman’s age).

While this news may be reassuring for some, others may feel disappointed as there is nothing tangible that can be ‘fixed’. These women may find it helpful to look into some holistic therapies such as acupuncture, which is proven to have a positive effect on pregnancy outcomes, but also so they feel like they are being proactive and taking some control back in what can be a time of feeling very out of control.

We would advocate that if you have experienced two or more consecutive miscarriages to seek medical advice and request some basic testing. This is especially pertinent if you are over the age of 35.

Let’s look at some of the most common causes of miscarriage. It can appear very overwhelming and even a bit scary, but please know that even with a positive diagnosis of one or more of these issues, you can still go on to have a healthy pregnancy with the right treatment, care and support.

Chromosomal Abnormalities

Approximately 60-70% of miscarriages in the first 12 weeks are due to major chromosomal abnormalities. In fact, 9 out of 10 genetically abnormal pregnancies will not survive past 12 weeks. These are generally one-off random occurrences and will not recur – unless advanced maternal age or compromised egg quality is at play.

Balanced Translocations

Balanced translocations are where a chromosomal abnormality is carried by either of the partners, and if passed on, will cause the fetus to be abnormal. The partner may not have any signs or knowledge of the abnormality until such time as a comprehensive blood karyotype test is done. In these cases, some couples may choose to ‘roll the dice’ and try naturally until such time as a pregnancy is successful. The other option is to undertake Preimplantation Genetic Diagnosis (PGD), an extended form of IVF whereby each embryo is tested for the particular translocation and effectively weeded out.

Uterine Abnormalities

Sometimes the uterus can have physical anomalies that can affect our ability to carry a baby. These include an irregular shaped uterus, a septate uterus or uterine fibroids. These abnormalities can generally be detected by having a hysterosalpingogram, an outpatients x-ray procedure. In some cases, minor surgery will be needed to correct an abnormality.

The lining of the uterus also plays a very important role in housing the embryo and a few women will have thinner than normal endometrium lining (optimum is 6mm and above in thickness). Again, once diagnosed, this can be relatively easily treated with hormone medication.

Blood Clotting Disorders (thrombophilias)

Blood clotting disorders can either be genetic or develop with age and Antiphospholipid antibodies can cause the blood to clot more easily and effectively cause a blockage in the forming placental blood vessels. Another condition called Factor V Leiden is an inherited version of thrombophilia and can contribute to reduced blood flow to the baby later in pregnancy, which needs to be closely monitored.

Auto Immune Disorders

Put simply, this is where our immune system becomes confused and attacks its own tissue – ie the body sees the pregnancy as an invader and attacks it. One of the most common immune disorder is Antiphospholipid Antibodies (ANA), though there are many different types of immune disorders that can affect pregnancy. Again, blood tests can determine if you are affected by any of these issues.

Natural Killer Cells

Immune cells or ‘Natural Killer Cells’ are actually important for detecting foreign cells in the body such as infections or cancer. However, some women can have higher than normal levels of these NKCs which can in turn cause the body to attack the seemingly foreign fetus. These NKCs can be found in either the blood or the uterus or in some cases, in both. Once detected, treatment for NKC is quite simple and very effective.

Hormonal Issues

  • Low Progesterone – medical practitioners can tend to have differing views on this one, ie, one school of thought is that low progesterone can cause miscarriage, while others believe that the progesterone is low because the pregnancy has not implanted successfully. Certainly when undergoing fertility treatments such as IVF, progesterone supplements are given as a matter of course, because in an artificial cycle, hormones need to be supplemented until such time as the placenta can take over.
  • Follicle Stimulating Hormone (FSH) – women with high FSH levels are more likely to experience miscarriage because their ovaries have become prematurely menopausal.

Maternal Age

While we may not feel old physically, unfortunately when it comes to fertility and miscarriage, our eggs play a huge part in the likelihood of success. Advanced maternal age is generally defined as being 35 or older, though most people would agree that 40 or older sees a much bigger decline in both ability to become pregnant and of course the chance of having a healthy pregnancy. That’s not to say it won’t happen! There are plenty of examples of women over 40 easily conceiving and having healthy babies. But when talking about miscarriage, the risks are certainly higher and the chances of early pregnancy loss more likely.

Comprehensive Testing

Usually after 2-3 consecutive miscarriages, your GP or OB/GYN or fertility specialist will send both you and your partner for comprehensive testing. Below is a list of what is generally tested:-

  • Male and female karyotype
  • AMH, LH, FSH, oestradiol
  • TSH Prolactin, Testosterone
  • Full Blood Count
  • Cardiolipin antibodies, Lupus anticoagulant
  • Protein C, Protein S, anti-thrombin III
  • APCR
  • Factor V Leiden
  • Prothrombin gene mutation
  • MTHFR mutation
  • Fasting glucose, insulin, homocysteine
  • Thyroid antibodies
  • Peripheral blood activated natural killer cells
  • Uterine natural killer cells and other endometrial pathology
  • Pelvic ultrasound scan
  • Hysteroscopy (and sometimes laparoscopy)
  • Semen analysis and sperm DNA fragmentation

Related Links

Miscarriage Support Downloadable Brochures
What is Miscarriage?
Signs and Symptoms
Treatment and procedures