Nobody knows. It was with no exaggeration that Lord Justice Judge, the deputy chief justice, said medical science was "still at the frontiers of knowledge" about unexplained infant deaths, when giving the appeal court's reasons for its decision to quash Angela Cannings' conviction for murdering her two baby sons.
Sudden Infant Death Syndrome (SIDS), known as cot death, is the leading cause of death in babies over one month old. "It takes more lives than meningitis or leukaemia or any form of cancer," says Joyce Epstein, director of the Foundation for the Study of Infant Deaths. "More babies die of cot death than are born with spinabifida or any range of illnesses. The peak age is 2-4 months. It can happen in much older babies, even over a year, but the cot-death rate drops off dramatically after 6 months." In 2002, 342 babies died in unexplained circumstances.
Finding out why is not simple. "In terms of research, it's a difficult thing to do because you don't know which babies are going to die," says Lucilla Poston, a foetal health researcher at King's College, London. "So it has to be epidemiological studies that do populations and correlations with lifestyle and environment."
Research has led to successes in reducing cot deaths. "Until about 1989, there were about 2,000 deaths a year. Then in 1991, we were able to launch a campaign advising parents that it's safer to sleep babies on the back than on the front," says Epstein. "Within one year, there was a 50% drop in cot death. That doesn't say anything about the causes of the death, of course, that just talked about risk factors."
There is also research on why SIDS happens in the first place, which focuses on areas including respiratory and cardiac systems, genetics, immunology and toxicology.
Genetics is a very promising avenue in trying to warn some parents that their child might be at risk. "There is no gene associated with cot death, but people are now looking at family histories and seeing if they can identify families at risk," says Poston.