Her best-selling book The Billings Method, first published in 1980, was translated into 22 languages and reprinted 16 times with seven new or revised editions.

Dr Billings' death was announced by WOOMB International, the organisation the couple set up to promote their method.

For half a century, WOOMB said, the Billings had travelled the world teaching and promoting their method in faithfulness to Pope Paul VI's call to scientists and physicians to "be obedient to the Lord's call and to act as faithful interpreters of his plan".

"In China alone, where they trained thousands to teach their method, a substantial drop in the abortion rate has been attributed to their work," WOOMB said.

A personal friend of three popes, Dr Lyn Billings was made a Dame Commander of St Gregory the Great by Pope John Paul II in 2003.

In 2002 the Australian couple were jointly declared International Catholic Physicians of the Year by the International Catholic Federation of Medical Associations.

Dr John Billings, who died in 2007, discovered the relationship between cervical mucus and fertility in 1953 while assisting the Melbourne Catholic Family Welfare Bureau.

Some of the couples he worked with had serious reasons to postpone pregnancy, and followed the Catholic Church's teaching of only using natural methods.

While Dr Billings was familiar with the Calendar and Basal Body Temperature methods, he felt there was a need for something more flexible and more reliable.

In a study of medical literature, he found mid-19th- and early-20th-century references to cervical mucus and sperm survival.

He instructed women using the Rhythm method to avoid intercourse on all days they noticed vaginal discharges, and for a few days after. This resulted in a dramatic decrease in unintended pregnancies among these couples.

By the late 1960s, the rules for identifying fertile days had been established and teaching centres for what was initially called the Ovulation Method began to be set up around the world.

Sources:

WOOMB International

Billings Ovulation Method (Wikipedia)

Image: WOOMB