Is Natural Family Planning supported by science? Yes! NFP relies on the science of a woman’s body. You could say that Natural Family Planning fits all the pieces together.


All natural family planning (NFP) methods have the same aim: Identify the times when a couple is fertile or infertile in order to know when to achieve or avoid pregnancy. This does not make NFP a method of contraception, since nothing is being done to prevent conception. The sex act is left undisturbed, and the spouses’ bodies are unaltered. A couple decides when to have intercourse based on their desire to postpone or achieve pregnancy.

A male is always fertile, but without the right conditions in the female body, his sperm will die within 20 minutes after completion of intercourse. A woman, on the other hand, cycles between being infertile and fertile all within an approximately month-long period.



Because males are always fertile and females are only cyclically fertile, NFP methods focus on understanding when a female’s body creates an environment hospitable to sperm, making conception possible. This is done by observing one or both of two measurable and quantifiable secondary characteristics of a woman’s body that occur as a result of the hormones that control her reproductive cycle: her basal body temperature and her cervical fluid discharges. In short:
An increase in cervical fluid indicates cycle onset of fertility.

A rise in basal body temperatures (which are correlated with a “drying up” of cervical mucus observations) signal a return to infertility after ovulation.
Using these two observations together, a complete picture of the fertile window leading up to ovulation and after ovulation can be made.

Each NFP method has the user identify her cycle peak day from her fluid observations, which is the last day that the most fertile fluid was observed. [Cervical fluid discharges closely resemble nasal mucus and come from similar crypts within the cervix. These fluids are commonly referred to as “cervical mucus,” so I will use that term from here on out.] Peak day is the highest statistically likely day of ovulation. (Note that the only way to identify ovulation with 100% certainty is through ultrasound. However, since we are interested in days of fertility or the “fertile window”, not just ovulation, determining ovulation day with 100% certainty is not necessary to use NFP.)

Identifying the onset of changing fertile mucus and peak day are the cornerstones to almost all NFP methods

A woman will ovulate during a 24-hour window of her cycle when follicle stimulating hormone (FSH – not shown on the chart) and luteinizing hormone (LH – green on the chart below) reach sufficient levels. A released egg has only 24 hours of viability; if it is not fertilized by a sperm during that time period, the egg will die and be absorbed back into the system.
However, the male’s sperm can live up to 5 days, depending on sufficient, good quality cervical mucus. Because of this, the days immediately preceding and the day following ovulation are potential days of fertility. Each NFP method has rules (based on significant research) that dictate how long after peak day abstinence is required for avoiding pregnancy.

Natural Family Planning is the Science of You


All NFP methods require the woman to make routine observations about her body, in order to apply the rules to achieve or avoid pregnancy. The two main types of NFP are the mucus-only methods, which record mucus observations, and the sympto-thermal methods, which record both basal body temperature and mucus observations. While observing and charting is a highly individual task, it helps to follow standardized rules that ensure the best possible observations. Some NFP methods are more standardized than others, so it is best to pick a method that will suit your needs. Some things to remember when making your observations:

  • When using basal body temperature (BBT) readings, it is important to take your temperature at the same time in the morning before getting out of bed. BBT data is based on circadian rhythms that are affected by hormones that start flowing in your body before you get out of bed. Data is best compared at the same time every morning when your body is still as close to being asleep as possible, before it starts to “heat up.”
  • It is important to use a basal body thermometer to ensure accuracy, not just any thermometer. Additionally, most thermometers have a “memory recall” button that allows you to take your temperature without needing to write it down right away. This means you barely have to wake up to take your temperature and can easily go back to sleep.
  • Cervical mucus is to be observed every time you void (both before and after) and right before you go to bed. This is as simple as wiping from front to back using a flat sheet of folded toilet tissue and examining what you see. You will be looking for two things: sensation when wiping and characteristics of the mucus. Both are important. A mucus observation that includes any of the following would be considered the most fertile (for identifying peak day): a wet or lubricative feeling when wiping, mucus that is clear or stretchy (think: egg white). Other cloudy, tackier mucus (think: more like tooth paste or pudding) will be less fertile. You will record the most fertile reading of the day on your chart. The NFP method you have chosen will have different ways to classify this and different rules for how to proceed.
  • In the pre-peak phase of your cycle, it is necessary to get a good mucus reading throughout the day before you can conclude if a day is fertile or infertile. This means that for almost all NFP methods, intercourse in the morning during the pre-peak phase is not advised.
  • It is important to identify true menstruation, which only follows ovulation. Not all bleeding is menstruation, including bleeding while using the Pill. Mucus observations must be made on days of light bleeding. Couples that are going off of hormonal contraception are advised to practice abstinence during the pre-peak phase for the first few months, in order to properly practice mucus readings and identify peak day (fertile mucus will closely resemble seminal fluid). Often what women think is their period is not, because they have not been ovulating while on the Pill.

There are several other things to keep in mind when making observations, and what I’ve given here is by no means a complete description. It is absolutely necessary to consult an NFP teacher who will be able to explain the details of their method, and who will work with you one-on-one to help you understand your chart and any issues you might have.


In order to use NFP, it is not necessary to understand how the careful balance of hormones creates the woman’s reproductive cycle. However, it is interesting information, and it can help identify problems when a woman has irregular cycles and/or is not conceiving when desired. Understanding the balance of these hormones and seeking to correct any imbalance is the basis for NaProTechnology with Dr. Hilgers, whose mission is the study of reproduction (and avoiding artificial reproductive techniques).


The hormones that dominate a woman’s cycle are estrogen (estradiol – blue on the chart) and progesterone (red on the chart). The pre-ovulation/pre-peak phase (also called the “follicular phase”, as this is when a follicle is developing to release an egg), is dominated by estrogen. After menstruation and in the beginning of the cycle, the pituitary gland in the brain will produce follicle stimulating hormone (FSH) that will do just that – stimulate the follicles on the ovaries to start getting ready to mature and release an egg. This developing follicle (shown in the graph) will begin to produce estrogen, which increases the uterine lining, opens the cervix to admit sperm, produces mucus that will aid in transmitting sperm, and depresses the basal body temperature.

High enough levels of estrogen emanating from the growing follicle on the ovary will eventually signal the pituitary gland to release luteinizing hormone (LH) to trigger ovulation. Incidentally, this is the hormone measured by the Clearblue® Easy Fertility Monitor and ovulation predictor kits.


In the post-peak phase or “luteal phase”, the burst follicle on the ovary (now called the corpus luteum or “yellow body”) will release progesterone that causes the body to get ready for pregnancy. It will signal the pituitary gland to stop producing FSH (which is why you won’t ovulate beyond a 24-hour window), the cervix will close, mucus will dry up, endometrial lining will increase in preparation for implantation, and basal body temperature will increase. However, after two weeks the corpus luteum will begin to dry up if it has not heard from a fertilized egg to keep producing progesterone. If the woman is not pregnant, progesterone levels will drop, the endometrial lining will slough off, the woman will get her period, and the cycle will start all over.

This introductory article is by not meant to substitute for real, individual NFP instruction! It is offered only as a introductory resource for those inquiring into NFP. As mentioned, many doctors have been researching and working on natural family planning methods over the last several decades, so there are multiple NFP methods available. There is truly something for everyone in any circumstance. Because NFP requires users to make observations on their own bodies each month, it is a highly individualized method, unlike the rhythm method, which used rules based on the average woman’s cycle. Regular cycles are not a requirement to use NFP.


This article was written by Alison Contreras. It was originally published at Alison blogs at and can be contacted through her email