Modern methods of NFP are rooted in a solid scientific understanding of the variations in fertility throughout a woman’s menstrual cycle. NFP methods have been shown to be from 95% to better than 99% effective in preventing pregnancy when used correctly and consistently (perfect use). If a method is used as the average person does, that is, sometimes incorrectly or inconsistently (typical use), the percent of effectiveness in preventing pregnancy can still range in the eighties and nineties. By identifying days of fertility, practicing NFP can also successfully aid couples who wish to achieve pregnancy.