The onset of menopause has always been understood to be time-driven, in that the gradual shutdown of the ovaries and the cessation of menstruation routinely occurs when a woman is in her forties or fifties. But a new European study links the event's timing to a more surprising marker: the season of the year.

Spring ahead, fall back "There was a . . . peak [in the beginnings of menopause] after the spring equinox" -- the sun's crossing of the equator around March 21, heralding the start of spring in the Northern Hemisphere, said Janos Garai, a reproductive endocrinologist at the University of Pecs in Hungary. Conversely, he found, autumn represented a low point for the start of menopause. The findings, published in the journal Human Reproduction, were based on questionnaires completed by 102 women.

Data check Even Garai acknowledges the study was limited by its size and other factors. Only 70 of the women could remember the exact date in question. Further, the study relied on the participants' memories and included only women in the city of Pecs. "I am highly suspicious of the accuracy of the data," said Wulf Utian, executive director of the North American Menopause Society.

One Theory Still, Garai thinks he may be on to something. Studies on animals -- dogs, hamsters, deer and horses -- suggest a strong link between fertility and seasonality. "It has been shown that the length of day determines some hormonal peaks," he said. He and colleagues speculate that melatonin -- a powerful hormone secreted in response to light and darkness -- could be responsible for the spike in women entering menopause. Could melatonin levels be manipulated to influence phases of the human reproductive cycles, including menopause? Said Garai. "More studies need to be done."

-- Ranit Mishori