A new large-scale US study has found that women born in the spring and summer may have a higher risk of dying from heart disease later in life.
Carried out by researchers at Brigham and Women's Hospital and Harvard Medical School, and Harvard T H Chan School of Public Health, the new study looked at data gathered from 116,911 US female registered nurses to investigate the possible associations between birth month, birth season, and risk of death from cardiovascular disease.
The participants were aged 30 to 55 years at the start of the study and were followed for a period of 38 years. During this time, they were asked to complete a detailed health and lifestyle questionnaire every two years.
Seasons were defined using the standard definitions of spring (March 21 to June 20), summer (June 21 to September 20), autumn (September 21 to December 20), and winter (December 21 to March 20).
The findings, published in The BMJ, showed that after taking into account various influencing factors, those born in the spring and summer had a small but significant increased risk of death from heart disease, although not from all causes, compared with those born in the autumn.
The effect was particularly strong for those born from March to July.
The finding is in line with previous studies carried out in the northern hemisphere, including in the US, Sweden, Germany, Denmark, and Spain, which have consistently linked birth in the spring and summer with a higher risk of death from all causes and from cardiovascular disease later in life. The reverse pattern has also been found in the southern hemisphere.
However, unlike previous studies, no significant association was found between birth month and overall death.
It is still not known why birth season could increase the risk of death from heart disease, but differences in diet, air pollution levels, and availability of sunlight before birth and in early life have been put forward as explanations.
The researchers note that as an observational study, they cannot establish cause and effect, however, they say that the study "adds to the growing evidence suggesting that individuals born in the spring and summer have higher cardiovascular mortality than those born in autumn, but conflicts with previous findings on overall mortality."
"Further investigations are required to confirm current findings and uncover mechanisms of seasonal birth month effect in cardiovascular mortality," they conclude.