The Science of Fertility Awareness

A standard dictionary defines science as “systematic knowledge of the physical, biological or material world gained through observation and experimentation which has been critically tested and validated.”

Fertility Awareness meets this definition of science. It is a body of knowledge about human fertility which has been observed, tested and validated over the past 60 years. Thus, while fertility awareness has an associated “theology” and “anthropology,” its foundation is really scientific.

The science of fertility awareness is best appreciated through an understanding of what is termed the “fertile window.” The fertile window is the time frame during each cycle when a woman can conceive. The fertile window is only open for six days out of the cycle, the five days prior to ovulation and the day of ovulation itself. (1)


If a couple has intercourse during the “fertile window” then conception is possible; if a couple has intercourse outside of the fertile window, then conception is not possible. Thus fertility awareness is based upon recognizing the signs of a woman’s body which indicate whether or not the window is open. We call these signs “bio-markers” and various NFP systems use one or more of the following bio-markers to determine whether the fertile window is open or closed:

  • Cervical mucus: Cervical mucus is produced by glands in the cervix, the mouth of the uterus. The mucus nourishes the sperm and facilitates their motility, allowing them to swim from the cervix to the fallopian tube where fertilization occurs. Studies from the World Health Organization (WHO) have shown that after 3 months, 90% of women can correctly identify and categorize their cervical mucus. (2)
  • Day of the cycle: Researchers studying women with regular cycles and cycle lengths between 26 and 32 days have shown the chances of pregnancy are much less on days 1 to 7 of the cycle and increase significantly from days 8 to 19. The chances of pregnancy then diminish from days 20 to the end of the cycle. We call these kinds of studies “day specific pregnancy probabilities” and these probabilities are factored into several systems of fertility awareness. (3)
  • Temperature elevations: After a woman ovulates, her ovaries produce increasing amounts of the hormone progesterone. Progesterone elevates a woman’s body temperature. A sustained temperature elevation, known as a “thermal shift,” can be used as a marker indicating ovulation has occurred and the window closed.
  • Luteinizing Hormone (LH) surge: Ovulation, the maturation and release of the egg, is triggered by an abrupt rise in the LH hormone released by the pituitary gland. This hormone can be detected in the urine by an ovulation prediction kit. Detecting the LH surge can help couples achieve pregnancy and also can be used as a marker to define when ovulation has occurred and a woman is “post-ovulatory.”
  • Duration and intensity of breast feeding: Studies have shown that intensive breast feeding has a suppressive effect on the activity of the ovaries. This kind of intensive breast feeding is part of a fertility awareness method called “Lactational Amenorrhea” or LAM. LAM provides specific guidelines for defining infertility while breast feeding.

There are a number of distinct methods of fertility awareness or natural family planning available. For the most part, the difference in fertility awareness methods come down to the number of bio-markers taken into account, the rules which guide the interpretation of fertile or infertile and the manner of charting or recording.

In addition to the actual methodology, the science of fertility awareness also examines the effectiveness of various methods and compares them to artificial means of family planning. The following links are a few examples of studies concerning the effectiveness of fertility awareness methods:

We can see from these and other studies that fertility awareness has a high rate of effectiveness and compares favorably to the effectiveness of artifical contraception.

The science of fertility awareness continues to be developed. Researchers continue to examine how the science of fertility awareness can be used in situations such as pre-menopause and in women with irregular cycles. The goal is to make the science of fertility awareness practical and accessible to couples, and to support them in this healthy and moral means of birth regulation.

1 Wilcox, A. J., C. R. Weinberg, et al. (1995). “Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby.” N Engl J Med 333(23): 1517-21.

2 World Health Organization, A prospective multicentre trial of the ovulation method of natural family planning. I, the teaching phase. Fertil. Steril. 1981a, 36:152-8.

3 Arévalo M, Sinai I, Jennings V Contraception. 1999 Dec;60(6):357-60.A fixed formula to define the fertile window of the menstrual cycle as the basis of a simple method of natural family planning.