By Elise Viebeck - 02/11/13 07:01 PM EST
The decision makes Benedict, who served for eight years, the first pope to resign since the 15th century.
His tenure was marked by global media attention toward sexual abuse by Catholic priests and a shift in the church's center of gravity toward the developing world.
Benedict, once the Vatican's doctrinal chief, also made plain his opposition to abortion, birth control and stem-cell research.
"It is necessary to help all people to be aware that the intrinsic evil of the crime of abortion, which attacks human life at its beginning, is also an aggression against society itself," the pope told Latin American bishops in 2005.
More recently, in January, Benedict praised the annual March for Life in Washington, D.C., and tweeted his support for participants.
Abortion has created friction between the pope and some U.S. leaders. Benedict notably raised the issue during a 2009 meeting with House Minority Leader Nancy Pelosi (D-Calif.), a Catholic who supports abortion rights.
According to the Vatican, the pope used the meeting to "speak of the requirements of the natural moral law and the church's consistent teaching on the dignity of human life from conception to natural death."
On Monday, Americans United for Life President Charmaine Yoest praised Benedict for "defending the unborn" and supporting "rights of conscience" when it comes to abortion.
"We are saddened that the Pontiff's declining health has led to his conscientious decision to resign his ministry," Yoest said in a statement. "He has been a source of inspiration to men and women of good will around the world."