Does anyone have any info. on this? Is it a catholic practice? Does it work?

http://www.bygpub.com/natural/natural-family-planning.htm

http://familydoctor.org/126.xml

"NFP is not "Catholic birth control". Nor is it the old rythmn method, which was the only way of spacing births when Humanae Vitae appeared. It is a method whereby a couple who have a good reason to wait for their next child exercise restraint during the wife's fertile period, which is determined by a few simple symptoms. " courtesy of http://www.catholic.net/RCC/Periodicals/Issues/NFP2.html