A study of nearly 3,000 post-menopausal women showed that the earliest age of menopause was found in women born in March and the latest among those born in October. On average there was around 15 months' difference, with women born in October reaching menopause at over 50 years compared with under 49 years for women born in March.
Writing in Europe's leading reproductive medicine journal Human Reproduction, lead author Dr Angelo Cagnacci said that the findings reinforced the concept that prenatal environmental factors affected a baby's adult life.
"Mothers should be aware of this, considering that during pregnancy they are going to influence, not only the health of the newborn, but also the health and reproductive life of their child during adulthood," he said.
The study, at four university hospitals (Bologna, Ferrara, Modena and Parma), was carried out on 2,822 women who were at least a year past the menopause. It found that the mean age of menopause was 49.42 – 49.04 for spring-born women and 49.97 for autumn-born women.
Dr Cagnacci, associate professor of Obstetrics and Gynaecology at the University of Modena, said that they had controlled for factors likely to interfere significantly with the findings, such as age of menarche, weight, smoking etc., and independently of all these factors the age at menopause was conditioned by the season of birth.
He conceded that there were some limitations to the study – a possibility of recall bias, and it involved women referred to menopause centres rather than the general population. Also, the effect of seasons may differ in different geographic areas.
"Nevertheless, the data seem to suggest notable effects of the month/season of birth on the length of a woman's fertile life, further supporting a role for environmental factors in regulating adult reproductive life and the timing of its termination."
It was not known which seasonal environmental factors were capable of influencing timing of menopause via an action exerted before birth, or even before conception. The study speculated that modifications of environmental temperature and sunlight may influence foetal growth and therefore future reproductive capabilities; seasonally linked diet modifications or exposure to infections were other possibilities. Before conception, seasonal environmental factors influencing ooctye maturation may contribute to the development of defective embryos and weak adults.
"Our present data seem to indicate that women born in autumn develop better during their prenatal life and are born with a higher number of oocytes than women born in spring," said Dr Cagnacci. "An alternative explanation may be that early mortality is highest among children born in autumn, thus selecting the fittest for survival, although other studies do not support this hypothesis."
He said the researchers' view was that a prolonged reproductive period with a later menopause was an index of higher embryo quality and ultimately, their quality was passed on to their offspring.
The next phase of the research was seeing whether the season of birth of the women in the study influenced their psychological profile – affecting their susceptibility to conditions such as anxiety and depression, for example. "Results so far seem promising," he said.
 Season of birth influences the timing of menopause. Human Reproduction. doi:10.1093/humrep/dei040.
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