It's a fact of life that many couples are familiar with — women usually lose interest in sex as time goes by.

Now, not only has a study proven this scientifically, but researchers also hope that the information they have gathered will help couples to be more realistic about their future sex lives, helping many relationships weather a tough reality.

A study conducted at the University of Guelph in Ontario, Canada found that for every month a woman stayed in a relationship, her sexual desire would decrease on average at about 0.02 on a Female Sexual Function Index.  

Authors Sarah Murray and Robin Milhausen also found that it was the length of time a woman spent in a relationship which most influenced her sexual desire, rather than sexual satisfaction or the state of the relationship.

While the decline recorded might seem small, the study found that it contrasted with male desire which did not see any decline from month to month.

While Murray and Milhausen believe that longer studies need to be conducted, they felt that the results of their study were indicative of evolutionary theory which suggests that women's attention typically turns towards child-rearing over time, while male sex drive remains level in order to continue to produce offspring.

The scientists also agree that hormonal changes may also play a part in the change in sexual desire in a long-term relationship, but don't believe that's the whole story.

According to LiveScience, pharmaceutical companies are currently investigating the role of testosterone in the female sex drive and whether it can play a part in boosting it. So far, there has been no proof indicating that this will be the answer.

The bottom line is that couples need to consciously work at keeping the spark alive in their relationships.

"When an individual has had sex with their partner over the course of many, many years, it takes creativity and openness to keep things fresh and exciting," Murray explained to LiveScience.

She cautions that without enough data, it's best not to jump to conclusions, suggesting that results of older couples may differ from this recent study of younger couples.

Younger couples who are coming out of the honeymoon phase of their relationship might experience an initial decline in sexual desire, but there is a possibility that this is not permanent.