The tests could reveal a woman's fertility

It could also help older women decide whether they need to use contraception.

The increasing trend in the UK is for women to delay motherhood until much later in life.

However, when women are approaching the menopause, usually from their late 40s onwards, their fertility decreases sharply.

As women postpone marriage and families, these findings could lead to happiness and prevent heartbreak Researchers, SWAN study

This is because their stock of eggs is low - it may be hard to predict whether they will release an egg during any particular month, and at what point in the cycle they will ovulate.

A large study of US women may yield a formula that could help doctors tell women reliably about their chances of conception.

There are several hormonal tests which can currently help doctors build a picture of a woman's fertility, and say whether ovulation is occurring.

In particular, levels of a hormone called FSH help doctors assess a woman's "ovarian reserve" - essentially how many immature eggs remain in her ovaries.

Mass testing

However, the doctors behind the latest project tested hundreds of women as they approached the menopause.

Their study, dubbed SWAN, or Study of Women's Health Across the Nation, analysed daily samples of urine for the presence of key hormones.

By looking to see what happened to their fertility in the following years, they hope they have found a way to tell women in more detail about what is likely to happen to them.

They discovered that they could predict with some accuracy the pattern of ovulation in women based on a mathematical formula generated this way.

The researchers wrote: "The findings demonstrated the usefulness of objective, computer-based algorithms to describe the primary features of the menstrual cycle with reasonable accuracy.

"It could provide invaluable assistance to middle-aged women in determining their peak times of fertility.

"As women postpone marriage and families, these findings could lead to happiness and prevent heartbreak."

No promises

However, UK fertility experts said it was hard to do more than take a snapshot of a woman's current fertility.

Professor Alan McNeilly, from the medical research council human reproduction sciences unit in Edinburgh, told BBC News Online: "At the end of the day, every woman is an individual.

"You could tell them about probabilities, but not certainties."