The start of a woman's menopause could be influenced by a change in the seasons, researchers have suggested.
Menopause rates peaked in spring
A survey of over 100 women by experts from Baranya County Teaching Hospital in Pécs, Hungary, found menopause rates peaked in the spring and autumn.
Rates soared just after the equinoxes, they say.
Writing in the journal Human Reproduction, the researchers say the seasonal pattern may also affect the symptoms women suffer.
Of the 102 women whose questionnaires were analysed, 72 remembered the exact month that their periods stopped, while 30 could recall only the season.
Women were also asked about a wide range of lifestyle and environmental factors, such as diet and exercise.
Dr János Garai, who led the study, said: "We found that there was a high peak after the spring equinox and another, lower one, after the autumn equinox."
The researchers suggest the seasonal pattern - seen in studies of animal reproduction - may also influence human reproduction.
But they stress the timing of a woman's menopause is obviously also affected by the decline in a woman's lifetime supply of eggs.
Dr Garai added: "Seasonal variations of reproductive functions in wild animals are well known, and similar but not so definite seasonal trends have been described for humans.
"The seasonality we found seems to support the influence of environmental factors on female human reproductive functions even when they are declining."
Although the researchers did not look at what could have caused the seasonal patterns, they suggest melatonin, a hormone secreted by the pituitary gland, could play a part in the process.
Melatonin plays a role in the circadian rhythm, the body's internal daily clock.
Until recently it was thought that melatonin acted only through the pituitary gland in the brain.
But melatonin receptors have also been found throughout the reproductive system.
The Hungarian team say this suggests melatonin could also influence ovarian steroid hormone production.
Dr Garai said: "It is plausible that this process [menopause] is not just due to the ovaries no longer being able to produce developing egg follicles that provide an adequate hormone supply.
"Rather, it can be perceived as the ovaries - governed by several internal and external factors affected by climatic conditions such as length of day, temperature and humidity."
He said, while the increased frequency of hot flushes in the summer could be explained by the weather, seasons could have more subtle effects on menopausal symptoms.
"No one has ever looked into whether the time of first missed bleeding shows any correlation with the severity or the frequency of hot flushes, or with the proportion of women who have them.
"Gaining more in depth knowledge might eventually lead to more efficient ways of dealing with menopausal problems."
But Joan Pitkin, a consultant in obstetrics and gynaecology at Northwick Park Hospital in London, told BBC News Online: "This evidence is flimsy at best.
"The study was asking women to remember when their periods stopped, and retrospective studies aren't always altogether accurate."
She said an observational study, where a group of women were monitored to see when their menopause occurred, would be more robust.
Miss Pitkin said: "The diagnosis of menopause is made after women have had no periods for a year. A lot of women can go for six or nine months without a period, then have another one. It can be very erratic."