The good health calendar

Research has found that the month in which you are born can have a profound effect on your health in later life. Scientists can't explain all of the findings and some are just theories, but here Good Health reveals which diseases are most common for each month - and how you can chart your risk.

In many cases, genetic susceptibility can be 'switched on' by infection at the time of conception or birth. And different seasons bring with them conditions more likely to affect a pregnant mother and developing foetus.

For example, in winter colds and flu abound and people are likely to spend more time together indoors - swopping germs and coming into contact with pets and pollution such as secondary cigarette smoke.

We are also more vulnerable to certain viruses in winter, when immune systems are weakened by colder weather and lower consumption of nutritious fresh fruit and vegetables. Lower levels of sunlight with life-giving vitamin D can also be health sapping.

In summer, a different set of germs are on the loose and activities such as swimming at public pools are popular, making cross-infection more likely. Hormonal levels can also undergo seasonal variations.

Greater credence has been given to some of the theories by research in Australia which came up with similar results but for the opposite months, which ties in with the different seasons in the Southern hemisphere.

Here, Roger Dobson looks at some of the amazing findings.

January

Heart disease

Being born in the coldest three months of the year increases the risk of developing heart disease by around 24 per cent. It is thought that cold temperatures around the time of birth may lead to increased fat storage and heart problems in later in life.

Epilepsy

Research based on epileptic patients discharged from hospital showed that there were more born in January and December than other months.

The lowest risk was among those with September birth dates. Prenatal infections, birth complications and nutritional deficiencies are implicated.

Alzheimer's

Research on patients with dementia found there was a significantly higher risk among those born in January, February and March. It is thought that as yet unspecified environmental factors, possibly viral, have an impact on some babies while the brain develops.

February

Depression

A number of studies have found that there is a significant increase in the number of people born in the first quarter of the year, especially February, who go on to develop major depression. One theory is that it is the effect of lack of sunshine in the winter months.

Schizophrenia

Those with winter and spring birthdays also have a greater risk of mental illnesses. The risk of schizophrenia increases during February and April, and babies born in February are ten per cent more likely to develop the condition than those born at other times of the year.

One theory gaining support is that some cases of schizophrenia may be caused by mothers being deprived of sunshine in a winter pregnancy. Among other possible causes are infections, maternal hormones, nutrition and temperature, and sperm quality.

Obesity

Research based on men in the South-East of England found that those born in the first six months of the year were more likely to be obese adults. It was particularly true of February, especially after a cold winter.

It could be that the cold slows down the speed at which food is processed, leading to a buildup of fat, and that if a baby develops a slow metabolism it stays for life.

Twins

Some research suggests twins are more prevalent in winter and spring, which may be caused by seasonal hormonal changes around conception.

March

Autism

A study based on 22 years of data on young patients with autism found that there was a significantly increased risk among those born in March and August. Viruses or infections around conception or in the early part of the pregnancy could be responsible, impacting on the developing brain.

Narcolepsy

This sleep disorder is most common in those born in March, and least likely in those with a September birthday. People born in March are 45 per cent more likely to develop the condition. Researchers believe that something, possibly an infection or virus, in the environment may trigger the condition in those with a genetic susceptibility.

Behaviour

According to Swedish researchers, the season of birth may be connected with certain personality traits. They found that among adults with mood or anxiety disorders, those born between February and April had lower levels of the feel-good brain chemical serotonin. Low levels are linked to depression and anxiety.

Temperament

Women born between February and April are significantly more likely to have an impulsive temperament. Men born during the first three months are more likely to be persistent. Environmental effects on the womb at a critical time for foetal brain growth could be the cause.

April

Multiple Sclerosis

Patients diagnosed with MS are more likely to have been born in either April or October. Researchers say these findings point to something in the environment which works on the central nervous system at a particular time of the year - such as a fever which could hinder healthy nerve development.

Anorexia

Women with anorexia are more likely to be born between March and June, when there is a 13 per cent increase in the number of sufferers born.

Temperature changes or infections around the time of conception may be a cause.

Asthma

Babies born around April are three times more likely to suffer from asthma in their first year if their mothers were exposed to pollen in late pregnancy.

A study of 100,000 children found that maternal pollen exposure played a major role. It is thought that antibodies produced in response to pollen pass into the baby's bloodstream in the womb and interact in some way with the foetal immune system.

Arthritis

A study of 1,405 people in Norway showed that April was the peak month for women who went on to develop arthritis of hip and knee joints. November births had the lowest rates.

Low sunlight (vitamin D) exposure prior to crucial skeletal development in the womb is suggested as a possible cause.

May

Seasonal Affective Disorder(SAD)

One study has found that a greater number of people with seasonal affective disorder are born in May. Just why is not clear, although a substantial part of the pregnancy would have been over the winter months when natural light levels are low.

Parkinson's

American researchers found that those born in the spring were more likely to develop Parkinson's disease. One theory is that an as yet unidentified kind of virus may have an effect on the development of the central nervous system.

Dyslexia

Early summer birth accounts for 24 to 71 per cent of cases of dyslexia, according to research, which shows particular peaks for May, June, and July.

Environmental effects during the colder months of the pregnancy at a critical time in brain development could be to blame.

Death from heart disease

Austrian researchers have found that death from both heart disease and stroke is greatest among people born between April and June and lowest among those born between October and December. Changes in nutrition and temperature during winter pregnancies are among the theories.

June

Coeliac disease

Caused by an inability to digest gluten, it often results in bowel symptoms, low weight, and can lead to anaemia and osteoporosis. Research based on children established that the risk of coeliac disease was 40 per cent higher in those born during the summer compared with the winter. Researchers say the findings may point to an infection of some kind, or to diet, including gluten intake.

Learning difficulties

Children with moderate learning difficulties are more likely to be born in the summer. One possibility is that there was some kind of environmental effect during early pregnancy, which would have been in the winter months.

Birth weight

Those born in late spring and summer are lighter, and one theory is that exposure to low winter temperatures during mid-gestation period may slow development.

Expectant mothers are more likely to be indoors during the winter months and may be at risk of greater exposure to pollutants, including tobacco smoke. Babies born in July were on average 31g lighter than those born in January. Poorer winter nutrition may also play a role.

Intelligence

Summer-born children perform better on tests of intelligence, mathematical ability, reading comprehension and communication skills, according to researchers. Some kind of seasonal change which affects the environment inside the womb may be involved.

July

Down's Syndrome

A number of researchers have found that children born in July and the summer months are more at risk and only six per cent of Down's births took place during January and February compared to the 17 per cent average. A winter pregnancy effect may be at work.

Bones and teeth

Young women athletes born between July and October had a 25 per cent higher frequency of bone fractures. Another study of elderly hospital patients found that those born in July and the summer months were at greater risk of breaking a leg.

The number of fillings children have has been found to be greater among those born in July-October, or in the latter half of a year.

One theory is that they spent the early part of pregnancy in the winter when the mother would not have been exposed to as much sunlight and vitamin D, crucial for bone and teeth development.

Hodgkin's disease

July is the peak risk month for Hodgkin's disease, a cancer of the lymphatic system, according to a Danish study based on more than 1,000 patients.

Those born in July had a risk 21 per cent higher than those born at other times of the year. Seasonal infections are one theory to explain this birth-month effect.

August

Crohn's disease

People born in summer are at the highest risk of chronic inflammatory disease of the gastrointestinal tract. Researchers say possible explanations include seasonal infections or environmental factors around the time of birth.

Diabetes

August is the peak risk month for the birth of babies who go on to develop diabetes in adulthood.

Swedish scientists have looked at thousands of births over a period of 30 years and found distinct seasonal variations.

Alarge British study also found a significant peak in early summer birth dates among those with insulin-dependent diabetes. Maternal infections or allergens may have an effect on the growing baby.

September

Attention deficit hyperactivity disorder

Boys born in September have the highest risk of ADHD, say researchers in Boston. They suggest viral infections in early development may be to blame.

Rheumatoid Arthritis

People with RA are more likely to have been born between September and the end of the year.

One theory is that infections or maternal exposure to as yet unknown environmental factors may be implicated. Another theory put forward by Australian researchers is that seasonal differences in exposure to ultraviolet radiation may be involved.

Female Fertility

Stress can affect fertility and it is thought that a peak in fertility during the summer holidays, when stress levels are lower, may be one reason for the higher rates of conception among working women in August and September.

Sudden infant death syndrome

The risk of SIDS is highest for those born in September and lowest for April children. 'For those born in autumn there may be two components - a genetic or intra-uterine component independent of month of birth, and the interaction with the winter environment,' say researchers.

October

Eczema

Research based on 4,040 patients shows that the peak birth months for sufferers are October, September and August. One theory is that something in the environment in the autumn may act as a trigger in some people.

Multiple Sclerosis

Patients diagnosed with MS are born in greater numbers in October and April. Researchers say this indicates the presence of unspecified natural nongenetic factors affecting the nervous system at a critical time.

Cryptochidism

Aalso known as undescended testes, the peak risk months are October, September and November. Hormonal effects in the womb are thought to be involved.

November

Left-handed

The peak months for the birth of left-handed boys are November and September. Seasonal differences in the levels of hormones in the womb, especially testosterone, are thought to be implicated. In the summer, sex hormone levels are higher.

Congenital dislocation of the hip (CDH)

A study based on 1,000 newborn children found that those born in November had the highest risk of CDH. May be connected with factors affecting bone growth in the womb.

Allergy

A Dutch study based on 150,000 people showed that those born between November and January were at the greatest risk for cat and dog allergy. The lowest risk was in May. Researchers say the findings may be due to increased early exposure to pets by being indoors during winter months.

Hayfever

A Swedish study of 55,000 men reports that the highest risk for hay fever was among those born between November and May.

The prevalence of bronchial asthma was 26.2 per cent among children born between November and December, compared with a ratio of 17.3 per cent among children born during the rest of the year. Environmental effects on the growing foetus are suspected.

Hay Fever

December

Longevity

People born in October, November or December live a few months longer that the average. In Austria, those born in autumn lived seven months longer. Seasonal differences in infections or nutrition may be to blame.

Height

A study based on 1.2 million children has shown that those born in December are on average 2.2mm shorter at birth than those born in April. A 2.2mm difference at birth length leads to a 6mm difference at 18.

Researchers say that seasonal variations in prenatal exposures may affect height. It could also, they say, be related to the amount of light exposure experienced by the mother late in the pregnancy

Personality

Those born between December and February are more likely to grow up to be agreeable people.

Scientists found that this was the only personality trait in which there was a seasonal link, but as yet they don't know why.

Leukaemia

Research in America shows that there is a significant majority of winter births among children diagnosed with acute leukaemia. Researchers say the findings suggest some kind of infection may be involved around the time of conception or during early pregnancy.

Scientists found a different pattern in Australia, where the seasons are reversed.