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From the position of a woman's 'bump' to the movement of a suspended ring, there are many old wives' tales that claim to be able to predict the sex of an unborn child.

But now mothers-to-be may want to keep an eye on the world's weather, as experts have suggested that changing temperatures affect the sex of babies.

A Japanese study has found that more baby girls are born, compared to boys, when temperatures rise in the country.

A Japanese study has found that more baby girls are born, compared to boys, when temperatures rise in the country. It reinforces previous research that suggests temperature plays a role in the sex ratio - but some of the results contradict each other

'The recent temperature fluctuations in Japan seem to be linked to a lower male:female sex ratio of newborn infants, partly via increased male foetal deaths,' according to scientists from M&K Health Institute and Shimizu Women's Clinic, both in Hyogo, as well as the University of Copenhagen.

'Male concepti seem to be especially vulnerable to external stress factors, including climate changes,' they wrote in a study published in the journal, Fertility and Sterility.

Previous studies have also shown that the sex ratio is sensitive to factors such as temperature.

Researchers recently found that more girls are born in tropical countries than boys, but warmer periods in Northern Finland led to more baby boys being born, Smithsonian reported. 

This reveals that some studies seem to contradict one another.

Earlier this year, pollution in central Scotland was linked to a skew in the sex ratio in favour of girls.

Previous studies have shown that the sex ratio is sensitive to factors such as temperature. Researchers have found that more girls are born in tropical countries than boys, but warmer periods in Northern Finland led to more baby boys being born. A stock image of a pregnant woman is pictured

To come up with the latest results, experts in Japan studied birth records and miscarriages in Japan between 1968 and 2012.

Overall, more girls were born compared to boys and temperature fluctuations – particularly from a hot summer to a cold winter – saw the sex ratio become temporarily more pronounced.

The study claims that temperature fluctuations have influenced the ratio, but makes it clear that climate change may not be responsible for skewing the number of girls and boys that are born.

It does however seem to reinforce previous studies that have found that male foetuses are more susceptible to events that can stress pregnant women, such as extreme heat waves.

'Two climate extremes, a very hot summer in 2010 and a very cold winter in January 2011, showed not only statistically significant declines in sex ratios of newborn infants 9 months later in June 2011 and October 2011 but also statistically significant increases of foetal death rates immediately, in September 2010 and January 2011,' the researchers wrote.

Animal studies have shown that rainy periods lead to more male chimpanzees, while loggerhead turtles produce more female offspring in warmer waters. Temperature is also known to determine the sex of reptiles.

If the world is getting hotter and rising temporaries do favour the birth of girls, a change to the global sex ratio may happen one day. A study in 2002 found that 101 boys are born compared to 100 girls. 

Another study has found that climate change affects people living in the poor parts of cities (such as new York, pictured) than more affluent residents. This is because poor areas tend to be more cramped and are hotter than more desirable places, which may have wider boulevards or parks