Together with her husband, Dr. John Billings, she founded the method of natural fertility regulation that bears their name.
Her studies on breastfeeding mothers and women approaching menopause made a major contribution to the work.
The couple traveled the world teaching the Billings method for half a century.
Her non-fiction book "The Billings Method" was first published in 1980, and was reprinted 16 times with seven new or revised editions in 22 languages. The latest completely revised edition was published in 2011.
She was a member of the Pontifical Academy for Life.
She is survived by eight of her nine children, 39 grandchildren and 31 great grandchildren.
Dr. John Billings died in 2007.