claims about how contraception can negatively impact fidelity within marriage are supported by a 2021 study in of more than 2,000 married women, some of whom practised NFP within their marriage and some who practised contraception. The study found that women who practised NFP were 58% less likely than their peers who used contraception to get divorced. Methods There are three main types of NFP: the symptoms-based methods, the calendar-based methods, and the breastfeeding or lactational amenorrhea method. Symptoms-based methods rely on biological signs of fertility, while calendar-based methods estimate the likelihood of fertility based on the length of past menstrual cycles. Clinical studies by the Guttmacher Institute found that periodic abstinence resulted in a 25.3 percent failure under typical conditions, though it did not differentiate between symptom-based and calendar-based methods. Symptoms-based Some methods of NFP track biological signs of fertility. When used outside of the Catholic concept of NFP, these methods are often referred to simply as fertility awareness-based methods rather than NFP. The three primary signs of a woman's fertility are her basal body temperature (BBT), her cervical mucus, and her cervical position. Computerized fertility monitors, such as Lady-Comp, may track basal body temperatures, hormonal levels in urine, changes in electrical resistance of a woman's saliva, or a mixture of these symptoms. From these symptoms, a woman can learn to assess her fertility without use of a computerized device. Some systems use only cervical mucus to determine fertility. Two well-known mucus-only methods are the Billings ovulation method and the Creighton Model FertilityCare System. If two or more signs are tracked, the method is referred to as a symptothermal method. Two popular symptothermal systems are taught by the Couple to Couple League and the Fertility Awareness Method (FAM) with Toni Weschler. A study completed in Germany in 2007 found that the symptothermal method has a method effectiveness of 99.6%. A study by the World Health Organization involving 869 fertile women from Australia, India, Ireland, the Philippines, and El Salvador found that 93% could accurately interpret their body's signals regardless of education and culture. A symptohormonal method of NFP developed at Marquette University uses the ClearBlue Easy fertility monitor and cycle history to determine the fertile window. The monitor measures estrogen and LH to determine the peak day. This method is also applicable during postpartum, breastfeeding, and perimenopause, and requires less abstinence than other NFP methods. Some couples prefer this