Women who are born in the summer are likely to have fewer children than women born in other months, researchers have suggested.
Researchers looked at the number of babies women had
The team, from the University of Vienna, Austria, say the finding is true despite modern contraception use.
Writing in the journal Human Reproduction, they say the finding mirrors trends seen in populations in the past.
However, there was no link between month of birth and childlessness.
The researchers looked at more than 3,000 Austrian women aged over 45 who were on a national database.
They found that, on average, the number of children was lower among women of reproductive age born between June and August than in those born in any other month.
Women born in July had 0.3 fewer children per woman compared to those born in December.
Historic studies of women in Canada and Holland also found that summer-born women had fewer children.
Dr Susanne Huber of the University of Veterinary Medicine in Vienna. who led the research, said: "It indicates that despite the potential influence of modern life and the use of contraception, birth month effects on later reproductive performance are at least to some degree similar in pre-modern and contemporary women."
She told BBC News Online there could be many possible causes for the association between birth month and reproductive performance.
But Dr Huber added: "It may be the conditions during foetal development and early childhood which affect future fecundity.
"Somewhere in this early development, the conditions may affect the development of the reproductive system.
"In temperate zones the external environment varies with the seasons, so the conditions experienced early in life are the result of a variety of seasonally and socially varying environmental and maternal factors that may all affect early development, causing potential effects on later life events."
The researchers now plan to carry out further research to look at whether a woman's future fecundity is affected most during her mother's pregnancy or during her early life.
But Dr Mark Hamilton of the British Fertility Society said such seasonal reproductive trends, affected by climate and the availability of food, were more evident in animals than in humans.
"Hence some species such as the deer enter the rutting season where males develop sexual maturity and females go in to heat in the autumn, such that offspring are conceived at that time and birth takes place the following spring.
"In humans such differences will be small if they exist at all and with populations now more mobile and with access to food, warmth and medical care throughout the year are unlikely to be clinically significant."