Miscarriage, also known medically as a spontaneous abortion, is defined as the early failure of pregnancy before the twenty-fourth week of gestation. After this time, the sad occasion of foetal death is classified as a stillbirth.
Between 20 per cent and 80 per cent of newly fertilized eggs (zygotes) fail to implant in the uterine wall. This is known as pre-implantation wastage and occurs before conception is medically or legally recognized.
Of those zygotes that do successfully implant and start to produce a placenta, around 30 per cent miscarry - most before the mother is even aware she is pregnant. The next period may be slightly later and slightly heavier than usual, but that is all.
Sadly, of all the pregnancies that are recognized as such by the mother, approximately 15 per cent fail to continue beyond the first five months of gestation. Twice as many will threaten to miscarry, with spotting of blood and/or period-like abdominal pains.
After a mother has had one miscarriage, her risk of another is twice that of a woman who has not previously had a miscarriage. After two consecutive miscarriages, the risk increases so that around 30 per cent of future pregnancies miscarry. This still means there is a 70 per cent chance of a pregnancy continuing to successful delivery of a child.
The risks do not continue to increase with each subsequent miscarriage, even though it may feel like it to a couple who have suffered through six or more miscarriages, one after the other. The future chance of a successful pregnancy still remains at around 50 per cent unless a specific, recurrent abnormality is diagnosed. In this case, your own physician can give you a more accurate assessment of your chances.
Genetic causes of miscarriage
- The vast majority of miscarriages are due to a one-off genetic disorder of the egg or sperm which makes the continued development of the fertilized egg impossible.
- Usually, the special cell division (meiosis) is resulting in each sperm or egg having only half the genetic information of other body cells goes wrong.
- The egg or sperm may lack some vital genetic information or, more commonly, an extra chromosome or set of chromosomes is present. This often occurs immediately after fertilization when the second polar body fails to leave the fertilized egg.
- Research shows that chromosomal abnormalities are detected in up to 60 per cent of recognized miscarriages, 5 per cent of stillbirths and 0.5 per cent of all live births. These figures suggest that around 10 per cent of all recognized pregnancies (half of spontaneous miscarriages) are identified as genetically abnormal.
- These genetic accidents are usually just that - an accident, possibly induced in a single germ cell by fee radical attack, toxins or exposure to irradiation. Only if the condition is a hereditary one will the risk of recurrent miscarriage increase.
- Chromosomal abnormalities are not always incompatible with reproduction. Several recognized syndromes exist in which parts of chromosomes are missing or present in excessive numbers.
- It is estimated that one in every 20 babies is born with a congenital malformation and that 5 per cent of the population suffer from a genetic disorder.
- Most infants with birth defects are born to women with no obvious risk factors, so detection of future problems is not always possible during the pre-conceptual care period.
Increasing maternal age
It is natural for the risk of a miscarriage to increase as you get older. Research suggests the risk of miscarriage doubles between the twenties and early thirties, then doubles again between early and late thirties. This is caused by the increasing age of maternal egg follicles which have been present in the female ovaries since birth. As well as increasing the risk of miscarriage, increasing maternal age is also linked with the risk of producing a child with a genetic disorder.
Down's syndrome, for example, occurs when an extra chromosome 21 is present in foetal cells. The following table gives the estimated risk of having a child with Down's syndrome according to maternal age:
Mother's Age (Risks of a child
with Down's Syndrome)
20 (1 in 1,000)
30 (1 in 880)
32 (1 in 720)
34 (1 in 460)
36 (1 in 280)
38 (1 in 180)
40 (1 in 100)
42 (1 in 70)
44 (1 in 40)
46 (1 in 25)
48 (1 in 15)
50 (1 in 10)
Pre-conceptual care is especially important for women over the age of 35 years. Ensuring an adequate intake of antioxidant vitamins and minerals throughout life, but especially during the pre-conceptual care period, will help keep your risks of spontaneous genetic mutations to a minimum.
Women over the age of 35 years, and those who have previously had an affected child, are offered techniques that exclude genetic abnormalities in future pregnancies. These include amniocentesis (aspiration of amniotic fluid to examine sloughed foetal cells) and chorionic villus sampling (removal of a small piece of placental tissue for analysis).
Non-genetic causes of miscarriage
Of the 40 per cent to 50 per cent of miscarriages that are not caused by a genetic abnormality, many are of uncertain origin. The foetus starts to develop normally but then, because of some physical insult or deficiency, becomes deformed or is rendered incapable of further development. Possible causes include:
- Nutritional deficiency of a vitamin or mineral
- Bacterial or viral infection
- Hormonal imbalance
- Pre-existing maternal disease
- Maternal disease associated with pregnancy itself
- Maternal anatomical abnormalities
- Immunological incompatibility between the foetus and the mother
- Altered blood clotting mechanisms
- Drugs, including alcohol
Some of these are discussed briefly below to show how pre-conceptual care can be important, especially if you have suffered a miscarriage in the past.
- Deficiency of all nutrients, but especially of the B group vitamins, folate, essential fatty acids, calcium, magnesium and zinc are associated with an increased risk of sub-fertility and early miscarriage. Lack of these nutrients interferes with cell division and DNA replication, a process occurring at a tremendous rate during foetal development.
- Women who smoke have a 27 per cent higher chance of suffering a miscarriage than non-smokers. Even passive smoking, especially where the mother lives with a smoker, has been linked with 4,000 miscarriages per year in the UK.
Bacterial or viral infections
- Diseases such as rubella, chlamydia, anaerobic vaginosis, cytomegalovirus, etc., can result in miscarriage or congenital deformity if contracted during early pregnancy. Other infections, such as influenza, pneumonia, appendicitis, etc, can also trigger miscarriage, especially during the first three months of pregnancy. This may be a mechanism to protect the mother, whose immune system is naturally depressed during pregnancy, thereby interfering with her ability to fight infection. As physical stress increases the body's needs for many vitamins and minerals, intercurrent infections also induce a relative nutrient deficiency, especially in chronic (long-term) grumbling types of infection e.g. acute cystitis, vaginal bacterial imbalance. This can also trigger miscarriage.
- There is less certainty regarding miscarriage and hormonal imbalance. Some researchers believe an inadequate corpus luteum (the collapsed follicle from which the egg was recently released) is at fault. Early pregnancies can only continue developing if supported by high levels of progesterone hormone secreted by the corpus luteum. Basically, progesterone is needed to prevent shedding of the endometrium (womb lining) when the next period is due. Progesterone is also needed to suppress maternal immunity so that the foreign foetal tissue is not rejected. The corpus luteum is maintained by a hormone signal (HCG-human chorionic gonadotrophin) secreted by the developing placenta. Failure of the corpus luteum may occur because of a lack of sensitivity to HCG, or an innate inability to secrete enough progesterone. In some countries, e.g. France, USA, progesterone injections or suppositories are given after ovulation and throughout early pregnancy in women suffering repeated miscarriages. This may increase their chances of a successful outcome, but is not universally accepted as an effective treatment. Another hormone imbalance linked with early miscarriage is a high level of luteinising hormone (LH). This seems to alter egg maturation before it is released from the ovary, making future miscarriage more likely. Treatment with the drug buserelin, which works by suppressing secretion of FSH and LH from the pituitary gland, may help.
Pre-existing maternal disease
- Some common medical conditions such as diabetes mellitus, high blood pressure, thyroid problems and anaemia increase the risk of miscarriage if not carefully monitored and controlled. If you suffer from any of these conditions, consult your doctor before conception so your care can be planned.
- All drugs, including alcohol, those bought over the counter, those prescribed by doctors and illicit, recreational drugs are best avoided during the pre-conceptual care period and throughout pregnancy. Many drugs are linked with early miscarriage or foetal abnormality. Only essential prescribed drugs should be continued and then only after informing the prescribing doctor that you are intending to try for a pregnancy.
By following a pre-conceptual care program, the message is very much one of hope.