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Fertile Math | Catholic Life

Fertile Math

People don’t understand Natural Family Planning. One of the reasons that I wish more would follow Cardinal Sean and use the term Fertility Awareness is that “Fertility Awareness” combined with abstinence is not only a more intuitive way to understand the Church’s teaching, it is also helpful with understanding the reality of how NFP actually works.

Have you ever thought “NFP= surprise babies” or said something like “NFP was the best mistake I ever made?”

Or are you one of those who worries that NFP is misused, used with a contraceptive mentality, too effective at avoiding pregnancy, or over-taught to young couples who have no reason to use it?

If so, have you considered that perhaps you do not understand how NFP actually works? Yes, there can be the occasional purely miraculous surprise pregnancy with NFP. Yes some couples may struggle with separate (I cannot say “unrelated” because all of our life is interrelated) sins of selfishness while practicing periodic abstinence for the sake of avoiding pregnancy. But NFP itself inherently avoids both problems. How? Math.

The effectiveness of NFP is very closely matched to the determination of the couple to avoid pregnancy.

If one is very strongly motivated to avoid pregnancy, then it is likely that one will be willing not only to observe and faithfully chart multiple fertility signs, one will also be willing to accept significant abstinence.

If one desires to avoid pregnancy, but is somewhat less motivated, then one can skip charting in favor of methods that don’t require good records, chart poorly (noting observations on some, but not all days), track fewer fertility signs, or, the most classic of all: abstain less.

There are some people who are simply misinformed. They may mistakenly expect NFP to be 99% reliable for avoiding pregnancy without following the rules that allow for such a rate. Some examples:

The woman who is shocked to be pregnant when she thought she was being so conservative with NFP, even though she was not charting was probably either using the rhythm method or two-day method, or intuition, and while all of those are methods of NFP in the strictest sense, they are all methods where a “surprise” pregnancy shouldn’t be shocking.

The woman who is new to NFP and says that she “ovulated early” and was “not supposed to ovulate until 3 days later” than she did In reality, one has to chart cycles for 12 months before one can say anything about when one as an individual woman is “supposed” to ovulate. The whole “day 14″ thing is just a generalization. If one is depending on a theoretical view of when women generally ovulate,  then one is using the classic rhythm method and should expect a pregnancy within about 5 years of using it faithfully.

The woman who “must have ovulated twice, or something” This is the reason God invented thermometers. And cervixes. And patience. And fertility monitors. And diligent following of the rules of the BOM-based method if that is really what one wants to use. Though, if this is an issue for you then I can’t imagine why a woman with access to computers would want to only use one sign of fertility and ignore all the rest. But that is another issue.

The woman who carefully follows her 96% effective method only to find herself in the 4% who become pregnant This may be the result of starting  a method that assumes infertility for the first 6 days, and being one of the rare women who has cycles so short that this rule is inapplicable. Or it could be only making external observations of cervical fluid when you are a woman who really needs internal observations. Or it could be counting as infertile days prior to ovulation where there is no cervical fluid, even though it will start an hour later. These are not precisely the same as the previous cases, but the couples who choose to follow these methods need to be aware of the likelihood that they may indeed be the reasonable exceptions.

The man who does not know what rules were being followed, or even anything about his wife’s fertility cycles, he just knows that they were “using NFP to avoid” and she got pregnant These are always sad cases because they indicate the fact that, while NFP is never contraceptive, men can experience it in the same way that they experience contraceptive methods which are simply left to the wife to take care of. Men, for practical purposes, if you don’t understand the logic of where your wife is in her fertility cycle and why conception is unlikely, you should assume that it is likely.

Now, if you were one of the people on the other side, you should now have a slight inkling about what it is you were missing when it comes to assuming that NFP is selfishly overused. NFP requires a lot of dedication. In order for NFP to be used as an extremely effective tool for avoiding pregnancy, a couple must be willing to abstain a lot. Yes, NFP may be a good tool for avoiding pregnancy without a lot of work or abstinance for a few women for a few cycles, but eventually it takes both serious dedication to observing and charting fertility signs and significant abstinence.

If a couple does not have a good reason to avoid pregnancy, then it is highly unlikely that they will be willing to put in the diligent effort that it takes to practice NFP in a way that is highly effective. Not only is God always in charge of conception (or the lack thereof), fertility awareness + abstinence for avoiding pregnancy is specifically orientated toward being ineffective for those who do not have a serious reason to avoid pregnancy.

Ultimately the NFP equation of fertility awareness + abstinence =no pregnancy means that NFP is incredibly scalable in its effectiveness for avoiding pregnancy. Those who are seriously motivated to learn rigorous forms of NFP, and then continue to be motivated to chart faithfully and abstain as much as is necessary are incredibly unlikely to experience a surprise pregnancy. But those who are not willing to put forth the dedication and self-control will find NFP significantly less than “very effective” for avoiding pregnancy.

This is why it is incorrect to see NFP as either inherently ineffective or easy to misuse. NFP is both effective and challenging. And that is precisely why it is such a wonderful tool for a couple who seeks to follow the Church’s guidance in determining the number of children that is appropriate for their family.

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5 Responses to “Fertile Math”

  1. Rebecca @ TRH 06. Jun, 2011 at 6:55 pm #


    While NFP has been a TOTAL blessing in our lives, there have been times where it was down right hard to continue avoiding – even though we were in total agreement that it was not yet time. And while it was hard, I have to say, this is the part of NFP that goes un-talked about. It’s the marriage building/strengthening part. While yes, I can see where long periods of abstinence could cause stress in a marriage, if both parties are willing, it can also build trust and respect. In a way that is almost impossible to put into words.

  2. Katarina 07. Jun, 2011 at 11:47 am #

    Sometimes these things are only learnt through experience for example

    ” Or it could be only making external observations of cervical fluid when you are a woman who really needs internal observations. Or it could be counting as infertile days prior to ovulation where there is no cervical fluid, even though it will start an hour later.

    When my cycles return we will be sticking to phase III lol — :-)

  3. alison 10. Jun, 2011 at 7:55 pm #

    I do find that in the couples I teach, it is the ones that get pregnant right away that find the abstinence the most difficult later on (at least they voice it the most), perhaps because they really didn’t realize how hard it is. Hard but necessary for it to work, of course.

  4. Alina 31. Jul, 2014 at 1:22 am #

    Yes, I think some people eexpct overnight transformation from contraception to full marital chastity and openness to life. While there are stories of couples doing just that, this is not the norm for most people. It can be a gradual and slow process. Just like some people can quit smoking cold turkey, but most need a gradual process.Presenting the instant and total conversion as the norm discourages couples from even trying. Furthermore, this all or nothing attitude pressures couples into relying on the method before they are confident in it and can use it effectively.For us, the Catholic NFP promotional material came across as, Oh, they just want us to outbreed the heathens. And if we don’t want to do that, then they want us to be just roommates. We felt like the promoters were more interested in recruiting us to fight the culture war than in our marriage or in her health. No, thank you.The secular/non-sectarian fertility awareness promotional material was Hey, this is a healthy and effective alternative to contraception! And it’s good for your relationship, too! Ok, this sounds like something we might be interested in NFP IS centered on understanding and appreciating fertility and it needs to be presented it as a positive and healthy option to couples. Being positive also means being honest and realistic.

  5. Laurence England 18. Sep, 2014 at 1:34 am #

    Hi, as far as I know, Pope Paul VI, in line with his predecessors stated spacing of children, abstinence during fertile periods could be justified for a ‘just cause’.

    A just cause is not simply, ‘We don’t want children at the moment’.

    Therefore, it is an area in which culpability, sin comes into play.

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