From MSNBC:

Natural birth control well-received, study says
Using beads to keep track of fertility was effective, inexpensive option

"A "natural family planning method" in which women use a set of beads to keep track of their fertile days is effective and widely acceptable to women and their partners, according to a new study conducted at 14 sites in six countries.
"I'm really convinced that this is a method that needs to be part of an informed choice approach to family planning," Dr. Victoria Jennings, director of the Georgetown University Institute for Reproductive Health in Washington, DC, told Reuters Health. "Women have lots of options and they need to be aware of this one."

"Ninety percent of both men and women in the study said they felt the SDM was easy to use, and that they would recommend it to others."