Selenium is known to protect against chromosomal (DNA) damage by protecting the body against toxins and pollutants, and future research may demonstrate the importance of prospective parents having good levels of this mineral in the months before conception, when both sperm and eggs are maturing. In fact, researchers have already found that women who miscarry have lower levels of selenium in their blood compared with women whose pregnancies go to term (Br J Obstet Gynaecol, 1996; 103: 130-2).

The idea that toxin damage could be implicated in Down's syndrome has been borne out by a study in the wake of the Chernobyl nuclear disaster. Babies born nine months later showed a sixfold increase in cases of Down's syndrome (BMJ, 1994; 30: 158-62). Studies on animals have also shown that it is possible to damage the chromosomes by introducing a toxin (Jennings I, Vitamins in Endocrine Metabolism, William Heinemann Medical Press, 1972).

So the opposite may also be true: that you can protect your chromosomes from damage by minimising your intake of environmental toxins and making sure that you have enough antioxidants to fight unavoidable pollution (like traffic fumes).

Infections

Miscarriage can be caused by infections. Often, it is a one off situation where the mother has caught a severe infection during the early part of pregnancy, and it is unlikely to recur in a subsequent pregnancy. Or the miscarriage could be due to a genitourinary infection which needs to be treated before she conceives again to prevent another miscarriage.

Mr Ronnie Lamont, consultant obstetrician and gynaecologist at Northwick Park Hospital, believes that bacterial vaginosis may trigger miscarriage or premature birth. In a study he conducted with 800 women, he found that those with bacterial vaginosis had five times the risk of late miscarriage (16-34 weeks), and those who delivered early (24-37 weeks) also had the infection.

Other infections that can cause a miscarriage include chlamydia; cytomegalovirus (CMV); herpesvirus, which can reduce a man's sperm count (Alder MW, ABC of Sexually Transmitted Diseases, British Medical Association, 1984); mycoplasmas such as Mycoplasma hominis and Ureaplasma urealyticum; toxoplasmosis; and genital herpes, which can increase the risk of miscarriage by up to 25 per cent. This is why it is vital to be screened for all such infections before you embark on trying to conceive. If you have miscarried, it's also wise to get a check up and have any underlying infections treated so that you don't miscarry again.

Fertility drugs

One of the most commonly prescribed medicines for fertility problems is clomiphene citrate (Clomid), which is used to induce ovulation. Ironically, though it may increase a woman's chances of conceiving, it also increases the chances of a miscarriage by 20-30 per cent (Regan L, Miscarriage, Bloomsbury Publishing, 1997). It is thought that clomiphene can interfere with the womb lining, preventing the fertilised egg from implanting. Other techniques used to induce ovulation, such as gonadotrophin treatment, can also increase the miscarriage rate.

Another overlooked cause is overweight. It is known that obesity increases the risk of miscarriage (Regan, cited above). Problems with excess weight can also be linked with polycystic ovaries (PCO), which can make conception more difficult. Miscarriages are more likely to occur in women with this condition. However, in a study of women with PCO who were asked to change their diet, the rate of miscarriages dropped from 75 per cent to 18 per cent for the same women once they had lost weight (Hum Reprod, 1998; 13: 1502-5).