Pregnant lady holding belly while coooking in kitchen

Cause of miscarriage

It is thought that around 50% of all miscarriages are due to a chromosomal problem occurring within the embryo. In women who are older than 35 years of age and those who have older partners, the incidence of this happening is increased. The other 50% of causes is not always clear. If a mother has a pre-existing medical condition then the cause of miscarriage can sometimes be attributed to her diagnosis. The more common causes for concern and increased risk are lupus, diabetes or other endocrine disorders, infection, hormonal problems, or abnormalities with the uterus. Occasionally a blighted ovum is diagnosed where, in the very early days of conception, the embryo does not divide and multiply normally.

It is generally thought that miscarriages which occur within the first three months (1st trimester) of pregnancy are due to problems with the development of the embryo. Those which occur in the 2nd trimester of pregnancy are more commonly due to medical conditions within the mother. The single most important risk factor for miscarriage occurring is the age of the mother when she conceives. Essentially, older women have more miscarriages than those who are younger.

The trilogy of cigarette smoking, being overweight or obese and a maternal age of over 35 years are all known to increase the risk of miscarriage. Although it may be difficult to identify any or all as a definite cause, they are all known to be responsible for a range of problems for both mother and baby – from the time of conception to birth.

Chromosomal Abnormalities

The genetic inheritance each of us gains from our parents is unique. Every cell in the body has a defined role and carries all the genetic information which is necessary to support healthy life. Chromosomes are very tiny packages of DNA and very early after conception, cells start to divide and separate to become specific tissues and organs in baby’s body. In every normal human cell there are 46 chromosomes; 22 pairs from each parent with the 23rd becoming the chromosome which determines our gender. Every time a cell divides there needs to be an exact replica of each of the chromosomes in the new cells. If this doesn’t happen, or one pair is incomplete or develops extra chromosomes, then miscarriage often results.

Sometimes miscarriage occurs because of an abnormal sperm or egg cell. Chance plays a huge part in whether faulty chromosomes are the cause for miscarriage and are not under the control of either parent; they just happen.

Some couples experience repeated miscarriage as a result of chromosomal defects and choose to undertake fertility assistance. Embryonic screening can be done to identify the healthiest embryos to be selected for implantation. Pre-implantation genetic diagnosis is usually undertaken with genetic counselling. This is offered to couples prior to this process so they can make an informed decision about what they can do with their embryos which may have genetic abnormalities.

Collagen Vascular Diseases and Auto Immune Disorders

These occur when a woman’s body creates antibodies which are targeted towards her own tissues. In order for a pregnancy to continue, the mother’s body needs to perceive the embryo as not being a risk to her. Blood clots are created as a result of an overall increase in the mother’s antibodies and these lead to a decrease in the blood flow to the embryo. Treatment is available following accurate diagnosis when antibodies are detected in a woman’s blood. One of the more common and effective treatments is with low dose aspirin and a blood thinning medication, both of which need to be monitored carefully in order to not create additional bleeding problems.

Coeliac disease is also thought to be a major cause for pregnancy rejection. This is a disorder of the gut where the body is intolerant of the dietary protein gluten and is unable to absorb nutrients properly from food. The immune system may be sensitive to a pregnancy when the immune response has been activated by the woman eating certain foods.

Uterine Abnormalities

Occasionally a problem with the shape of a woman’s uterus leads to miscarriage. The embryo cannot implant in an optimum site to allow for healthy blood supply and growth. Although this may be sustainable in the early weeks of pregnancy, it cannot continue and miscarriage results. A septate uterus is a common type of congenital uterine anomaly, and it may lead to an increased rate of pregnancy loss. The main imaging differential diagnoses are arcuate uterus and bicornuate uterus. Surgery to correct uterine shape and size may be an option before trying to conceive again.

Diabetes Mellitus

In women who have well controlled diabetes, the risks of miscarriage are reduced. Diabetes in itself is not thought to cause miscarriage, though, when combined with other factors does increase the risk of not being able to continue with the pregnancy until term. Diabetes can lead to complications during pregnancy and delivery. Close monitoring is essential to ensure blood sugars are maintained within a healthy range and do not compromise the health of the baby. Babies of diabetic mothers are often larger at birth and require monitoring of their own blood sugar levels in the early neonatal period.


There is a range of bacterial and viral infections which, when caught by a mother in the first trimester of her pregnancy, increase the risk of her miscarrying. Some of these infections do not create any symptoms within the mother herself. It is currently thought that the main ways infections find their way into the uterus is via the placenta or ascending through the mother’s cervix. Infections can occur in the placenta itself or in the embryo. Occasionally blood borne infections are the cause. Certain types of bacterial food poisoning, such as Listeria and Salmonella infections, are also tied to miscarriage risks.

Some of the major infections are mumps, rubella (German measles), herpes, listeria and a range of sexually transmitted diseases. Very early in the first trimester, blood tests are taken to screen and assess each mother’s immunity.

Hormonal Imbalance

Some women experience what is known as a Luteal Phase Defect. This occurs when the ovary does not produce sufficient progesterone to support the embryo in the very early stages of conception. Though some researchers believe that low levels of this important pregnancy hormone are a symptom, rather than a cause of miscarriage. The balance of progesterone, follicle stimulating hormone, oestrogen and even thyroid hormones all need to be in the correct ratio to support early pregnancy. Too little or too much of any one of them can result in pregnancy loss.

Lifestyle Causes

Smoking cigarettes, drinking too much caffeine, taking illicit drugs, particularly cocaine, being exposed to radiation or toxic substances or drinking too much alcohol are also cited as causes for miscarriage. Interruption of a healthy blood supply to the embryo via the placenta also leads to problems with healthy cell division.


Occasionally trauma is responsible for causing miscarriage. Although the embryo is well protected within the bony pelvis and the lining of the uterus, if sufficient force is delivered, the placenta and embryo can sheer away and become detached. Motor vehicle accidents or sustaining force to the lower abdominal region can be the cause.

Symptoms of Miscarriage

These can vary between women, with some women experiencing all symptoms and others only very subtle physical changes.

  • Vaginal bleeding is one of the most early, common signs of miscarriage. This can be very slight with only a vague pinkish discolouration to vaginal mucous or a heavy, fresh blood loss.
  • Some women pass blood clots, especially after intervals of lying down. As they stand up they experience a gush of blood.
  • Abdominal pain with cramping similar to period pain, or back pain which comes and goes.
  • A lessening of pregnancy symptoms. This is usually one of the later symptoms of miscarriage when the woman’s body stops producing Human Chorionic Gonadotrophin (HcG) – the pregnancy hormone.

When Can We Start Trying Again?

If the miscarriage has been incomplete, if bleeding continues or there is an empty pregnancy sac detected on ultrasound, a dilatation and curette (D and C) of the uterus is scheduled. This is a relatively minor surgical procedure, done under general anaesthetic to clear the contents of the uterus of all products of conception. Most women will experience vaginal bleeding for a few days after this procedure until it settles down. For as long as HcG is circulating in a mother’s body, she will not ovulate or have a period. In the majority of cases, it will take around ten days for the levels of HcG to settle and for her body’s hormone levels to return to their pre-pregnancy state. Many will have a normal period in four to six weeks after a miscarriage; this is a sign that ovulation and a return to normal fertility have occurred.

Some health care practitioners recommend couples try to conceive as early as possible after a miscarriage. They feel there is no benefit in waiting and in the majority of situations, there is no increase in the likelihood of miscarriage occurring again. Others claim there are benefits to waiting three or four months and to allow a couple of monthly cycles to occur which may help to “normalise” ovulation and menstruation. Consistently, advice includes waiting until vaginal bleeding has stopped and the woman feels well enough, both physically and emotionally to consider trying to conceive again.

If a medical condition or infection were thought to be the cause for miscarriage, then becoming well and stable are essential. Waiting until hormone levels have stabilised, blood loss has settled and there has been a chance to restore healthy nutrition will optimise the chances of carrying a healthy pregnancy next time.

But I Feel So Sad

Miscarriage of a wanted baby can be devastating to many couples. Although it may only have occurred in the early stages of pregnancy, this does not mean it can just be passed off as something minor. Caring for the physical needs of a woman following miscarriage is relatively straightforward but it is important not to overlook the emotional side as well. Counselling is available to women who feel they need it. Check with your GP or health care practitioner.

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