It is highly unlikely that you will ever know the actual cause of a one-off miscarriage, but most are due to the following problems:

• ABNORMAL FETUS

The most common cause of miscarriages in the first couple of months is a one-off abnormal development in the fetus, often due to chromosome anomalies. ‘It’s not as though the baby is fine one minute and suddenly dies the next,’ says Professor James Walker, Professor of Obstetrics and Gynaecology at the University of Leeds. 

‘These pregnancies fail from the outset and were never destined to succeed.’ Most miscarriages like this happen by eight weeks, although bleeding may not start until three or four weeks later, which is worth remembering in subsequent pregnancies. ‘If a scan at eight weeks shows a healthy heart beat, you have a 95 per cent chance of a successful pregnancy,’ says Professor Walker.

• HORMONAL FACTORS

A hormonal blip could cause a sporadic miscarriage and never be a problem again. However, a small number of women who have long cycles and irregular periods may suffer recurrent miscarriages because the lining of the uterus is too thin, making implantation difficult. 

Unfortunately, hormone treatment is not terribly successful. 

‘There used to be a trend for progesterone treatment, but trials show this really doesn’t work,’ warns Professor Walker. ‘There is some evidence that injections of HCG (human chorionic gonadotrophin, a hormone released in early pregnancy) can help, but it’s not the answer for everyone.’ The treatment must be started as soon as the pregnancy is confirmed, at around four or five weeks.

• AGE 

For women over 40, one in four women who become pregnant will miscarry. [One in four women of all ages miscarry, but these figures include women who don’t know that they are pregnant. Of women who do know that they’re pregnant, the figure is one in six. Once you’re over 40, and know that you’re pregnant, the figure rises to one in four]

• AUTO-IMMUNE BLOOD DISORDERS

Around 20 per cent of recurrent miscarriers suffer from lupus or a similar auto-immune disorder that causes blood clots to form in the developing placenta. 

A simple blood test, which may need to be repeated several times, can reveal whether or not this is the problem.’One negative test does not mean that a women is okay,’ warns Mr Roy Farquharson, consultant gynaecologist who runs an early pregnancy unit at the Liverpool Women’s Hospital. 

Often pregnancy can be a trigger for these disorders, so a test should be done as soon as possible,’ he adds.But it can easily be treated with low dose aspirin or heparin injections, which help to thin the blood and prevent blood clots forming – a recent trial also showed that women do equally well on either. ”We have a 70 per cent live birth rate in women treated for these disorders,’ says Dr Farquharson, ‘which is excellent.’

• OTHER CAUSES

While uterine abnormalities, such as fibroids, can cause a miscarriage, many women have no problems carrying a pregnancy to term. An incompetent cervix can also cause miscarriage at around 20 weeks. 

While this can be treated by a special stitch in the cervix, trials suggest it is not particularly successful, although it may delay labour by a few weeks.Gene and chromosomal abnormalities, which can be detected by blood tests, may also cause recurrent miscarriages in a small number of couples. 

A procedure known as preimplantation genetic diagnosis can help. After in-vitro fertilisation (IVF), a single cell is taken from the developing embryo and tested for the gene defect. Only healthy embryos are then replaced in the womb.

 It is an expensive and stressful procedure – and pregnancy rates tend to be quite low – but for some this is preferable to repeated miscarriages or a genetically abnormal baby.