Astrology columns deserve nothing but your disdain, right? When you're born couldn't possibly make a difference to your chances with a lover or bank manager, could it?
Of course not. But there is evidence emerging that the season or month in which you were born may make you more - or less - susceptible to certain illnesses. A paper in this week's British Medical Journal describes how the incidence of multiple sclerosis (MS) seems to be related to the month of birth. The researchers looked at MS patients in Canada, Denmark, Sweden, and Lothian and the Borders in Scotland. They found significantly fewer patients with MS born in November, but significantly more born in May. There is already evidence about the incidence of MS that relates to where you are born. Essentially, the higher the latitude, the greater the incidence.
Professor George Ebers, of the Department of Clinical Neurology at Oxford University, who led the research, says: "We have known for a long time that there is something environmental that influences the risk of MS. If you compare northern and southern hemispheres, there is a gradient of incidence that increases as you move into higher latitudes - we see this in Australia, New Zealand, the US and the UK. We think that something is happening very early to influence this.
"The increased risk of MS if you are born in May does imply something in the environment and it may be something to do with sunlight or Vitamin D exposure in pregnancy. That would be a testable hypothesis - and one that we have started researching. Sunlight influences a variety of different cycles and hormones - for example, the pineal gland, melatonin, as well as temperature responses."
And there are other patterns of risk. Thomas Joiner, Bright-Burton Professor of Psychology at Florida State University, has been studying the effect of birth month on psychological symptoms experienced later in life. Although many factors are responsible for mental illness, his research shows an association with exposure to flu when in the womb.
Young adults born in the southern hemisphere between September and November (who would have been in utero during the hemisphere's flu-season peak four months earlier), showed a high level of depressive symptoms. In addition, he found that young Australians who had been born in the northern hemisphere between March and May (putting them in their second trimester during the northern hemisphere flu season) also had high levels of depression. "It's a step toward understanding biological risk for suicidal behaviour," says Joiner. "Flu is by far the most likely mechanism. Prenatal exposure to influenza affects neurotransmitter systems, patterns of cell connectivity, and central nervous system morphology. The second trimester of pregnancy is crucial, because the foundations for organisation of the nervous system - like cell birth, death and recovery - are occurring."
Joiner has replicated this research with people whose average age was about 60. "I think that's compelling - something that happened around 60 years ago still has some effect."
The association between winter births and schizophrenia is well known, being first reported in Switzerland in 1929. Since then, it has been reproduced by researchers in Europe and the US. Dr Brian Kirkpatrick first reported a different pattern of birth distribution in deficit schizophrenia, a subgroup of schizophrenia, characterised by social withdrawal and lack of motivation. He and Dr Erick Messias, a psychiatric epidemiologist, "pooled" patients from the US, France, Spain, England, Scotland, and Ireland. "Analysis showed that patients with this subgroup category of deficit schizophrenia were twice as likely to be born in the summer when compared with those with nondeficit schizophrenia," says Messias. "This finding strongly points to the idea that deficit and nondeficit schizophrenia are two different diseases, much like diabetes type 1 and type 2.
"It means that we need to start thinking of these patients as having a different disease, with different treatment needs, and a different cause. By finding a homogenous group we can tailor our treatment interventions, and improve the search for the causes of schizophrenia."
In Denmark, a study led by Dr Jan Wohlfahrt analysed more than a million children to find that children born in spring and autumn were tallest, and the shortest children were those born in December. The Max Planck Institute for Demographic Research in Germany has found that, in the northern hemisphere, people born in autumn live longer than those born in spring. But in Australia, this pattern shifts by six months, keeping the seasonal trend constant.
Dr Debbie Lawlor, senior lecturer in epidemiology at the University of Bristol, has a special interest in this field. "There are many historical studies that show that life expectancy, all-cause mortality, and cardiovascular disease mortality are all patterned by season of birth," she says. Working on the British Women's Heart and Health Study (a large study on the causes and consequences of heart disease, funded by the Department of Health), she wanted to find out if the weather conditions at time of birth influenced later illness.
Using a historical dataset produced by geographers at the University of Cambridge, she calculated the environmental temperature at time of birth for subjects born between 1919 and 1940. "Women born in winter were more likely to be obese, have insulin resistance [often a precursor of diabetes] and heart disease. One theory is that people who are born during cold weather seasons seem to lay down more fat, and could therefore be at greater risk of insulin resistance and heart disease. There is a move towards considering genes in terms of 'plasticity': the genes that put you at increased risk of obesity and heart disease may express themselves differently depending on what the risks of the environment, such as temperature, are at the time of early development."
Lawlor cites the book Season of Birth, written over 70 years ago by Ellsworth Huntington: "He says that, basically, human thermoregulation is crucial." But how much credence can she give to this season-of-birth-theory? "Epidemiology is plagued with the possibility of chance findings or confounding factors, and we do need more work to be done. But this kind of data on season of birth is reproducible, does fit with animal data and is plausible. If these findings are confirmed, then from the point of view of public health it is important to realise that there are things we can do about it.
"For example, we know that in Siberia, there are less extra deaths in adulthood caused by winter weather compared with the UK, because here, we don't protect ourselves enough. In our study, the effect of cold weather around the time of birth was stronger in those whose fathers were in manual social classes than those in non-manual social classes, suggesting that the better-off families were more able to protect themselves from the cold exposure."
And if we can't do much about when we are born, it might still hold the key to understanding, targeting and limiting the risks to future generations. It's certainly no excuse for blaming your horoscope.
