Last week Sally Clark was convicted of murdering her two babies. She said they were 'cot death' victims. How many other parents have done the same?

Last week, Sally Clark was convicted of the murder of her two sons, Christopher and Harry. One died at the age of 11 weeks, the other was eight weeks old. Their mother claimed that the two babies were victims of cot death, but a post-mortem showed that the younger child had been shaken to death. Mrs Clark, who suffered from alcoholism and depression, is now awaiting sentencing.

Last week, Sally Clark was convicted of the murder of her two sons, Christopher and Harry. One died at the age of 11 weeks, the other was eight weeks old. Their mother claimed that the two babies were victims of cot death, but a post-mortem showed that the younger child had been shaken to death. Mrs Clark, who suffered from alcoholism and depression, is now awaiting sentencing.

The Clark case has attracted particular attention, partly because of the double death, but also because of the ostentatiously affluent background of the family. Mrs Clark, 35, and her husband Stephen are both solicitors, they live in a prosperous and expensive suburb, they employed a nanny ­ an unlikely backdrop for child murder. As the detective heading the investigation, Detective Inspector John Gardner, commented: "It is harder to accept when it's a woman who has had all the advantages."

In recent years there have been other notorious cases of mothers who have killed their infants: Caroline Beale, the woman who gave birth alone in a hotel bath during a holiday in the US, and was arrested at the airport carrying the body in a bag hidden under her coat; Melissa Drexler, the so-called "prom mom" from New Jersey who had her baby in the lavatories during a school dance, put the newborn in a rubbish bin, and went back to the disco.

Alongside the cases that make the headlines, there are others that barely merit a mention. In the same week that Sally Clark was found guilty, a 15-year-old appeared at Nottingham Crown Court after giving birth secretly and stabbing her newborn baby. Earlier this year, a 37-year-old nurse battered her four-month-old daughter to death and left the body in a rubbish chute. Also this year, a 14-year-old drowned her newborn daughter before hiding the body in her school bag with her favourite teddy bear, and another teenager was put on probation for smothering her five-month-old baby.

Rather than murder, which is the charge that Sally Clark chose to defend, the mothers in these cases faced accusations of infanticide. Such charges can be brought where the child is under a year old and it is thought that the mother is suffering from mental problems caused by the birth. By pleading guilty to the lesser charge, the mother may receive a probationary sentence.

Babies under a year old are the sector of the population most at risk of homicide, according to Home Office crime figures ­ around 35 deaths a year. Dr Margaret Oates, a perinatal psychiatrist at Queen's Medical Centre in Nottingham, says that these mothers can be divided into two groups. The first are typically very young and likely to have concealed their pregnancy; these are the neonaticides, who account for around a quarter of baby deaths. "They give birth alone and the baby dies within hours," says Dr Oates. "They are often nice girls from decent families who are not terribly streetwise." Some, she says, can disassociate themselves completely from their pregnancy. "They give birth without making a sound and carry on as normal afterwards. In one case, the girl was playing netball within two hours of giving birth to an 8lb baby."

The rest, says Dr Oates, are a varied group. "Sometimes the mother is psychotic, in which case she often tries to kill herself, too. And there is a small group who will kill babies for no apparent psychiatric reason." Dr Maureen Marks of the Institute of Psychiatry says that, neonaticides apart, most children under the age of one are killed in a sudden fit of temper. "Young babies elicit very strong feelings, both of joy and of anger," she says. "Parents who come from a family background of violence where people give way to their feelings are more likely to give way themselves. And parents who experience a sudden loss of temper are often suffering from chronic sub-clinical depression."

It seems that there is little to be done to prevent such cases. Dr Marks points out that most people who are depressed do not harm their babies. She adds that the best solution is early intervention, way before violence becomes a potential problem ­ initiatives like screening new mothers for depression and offering counselling. "Infanticide is such a rare event that you cannot expect people to spot it beforehand," Dr Oates says.

Sally Clark's husband says that she will be appealing against her conviction. Mrs Clark, who gave birth to a third baby boy while awaiting trial, has consistently claimed that both her earlier babies were victims of cot death, or Sudden Infant Death Syndrome (Sids) ­ that she had found them dead for no apparent reason. In such an affluent family the odds against this happening twice have been calculated at 73 million to one. Christopher's sudden death while he lay in his Moses basket at home was initially put down to a respiratory tract infection, and the Clarks were given counselling for cot death. Suspicions were only raised when Harry died in January of last year. Mr Clark is still clinging to the notion that both his sons died of natural causes, and is appealing to other families who have suffered multiple cot deaths to come forward.

Until Sids was officially accepted as a cause of infant death at the beginning of the 1970s, the deaths of babies who died for no obvious reason would be recorded under diagnoses so generalised as to be virtually meaningless ­ or put down to inadequate care, which was extremely distressing for the parents. However, some researchers believe that the diagnosis of cot death may be made too freely in some cases. Research at Sheffield University earlier this year found that of 29 child deaths put down to Sids, 11 would have been better classified as "unascertained" because of lack of evidence. Another study, carried out at St James's Hospital in Leeds, found that of 81 cases of children killed by their parents, 49 had originally been certified as cot deaths.

Writing in a recent issue of the British Medical Journal, Michael Green, emeritus professor at the department of forensic pathology at the University of Sheffield, says: "In private conversation, many paediatric pathologists and forensic pathologists say that parental or adult intervention may have occurred in 20 to 40 per cent of the cases of so-called sudden infant death syndrome with which they are involved." Professor Green notes that the "back to sleep" campaign run by campaigners against cot death, which encourages parents to lay babies on their backs, has helped to cut cot-death cases from around 1,000 per year to around 400. "If we assume (and to me it seems a reasonable assumption) that the numbers of adults who harm their children have remained fairly constant over the years, then in the relatively low number of baby deaths currently occurring, the proportion of 'suspicious' cases is increased accordingly. It follows that all of us involved in such deaths should approach them with suspicion, albeit cautiously expressed."

The Foundation for the Study of Infant Deaths finds such an approach "retrograde". "We feel very strongly that every death should be thoroughly investigated, not just if there is something suspicious ­ parents want to know exactly how their child died," says Dr Sara Levene, medical advisor to the foundation. "There are cases where nothing at all can be found, and Sids is a very helpful label in these cases." Dr Levene added that some of the headlines connected with the Clark case had been "very distressing". "If people stop believing cot death is genuine, it will be very sad for parents and families who have suffered." The success of the "back to sleep" campaign, she says, proves that cot death is a genuine condition. "A very occasional case of child abuse may slip by but the overwhelming majority diagnosed as cot death are cot death."

However, Professor Green, who has given evidence in criminal cases where smothering was alleged, says: "I believe that the pathologist has a place in any 'care of next infant' scheme." It is, he says, a "chastening experience" to perform a post-mortem on a child victim of homicide whose elder sibling was signed off as a cot death a few years previously. "I should know," he says. "I have done it."