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The first formalized calendar-based method was developed in 1930 by John Smulders, a Roman Catholic physician from the Netherlands. It was based on knowledge of the menstrual cycle. This method was independently discovered by Hermann Knaus (Austria), and Kyusaku Ogino (Japan). This system was a main form of birth control available to Catholic couples for several decades, until the popularization of symptoms-based fertility awareness methods. A new development in calendar-based methods occurred in 1999, when Georgetown University introduced the Standard Days Method. The Standard Days Method is promoted in conjunction with a product called CycleBeads, a ring of colored beads which are meant to help the user keep track of her fertile and non-fertile days. wiki

The rhythm method is the only form of birth control officially accepted by the Catholic church (I am also including the updated version called "natural family planning" under this umbrella) and one of the main reasons for it still being practiced (and apparently modestly increasing in popularity) is listed as "morals" by the CDC.

About 17 percent of sexually experienced teen girls told researchers for the Centers for Disease Control and Prevention that they had used the rhythm method -- timing their sex to avoid fertile days to prevent getting pregnant. That figure is up from 11 percent in a similar survey in 2002. Washington Post

This method does not use any drugs or barrier devices to stop conception, and has a few variations, which can involve charting calendar dates of menstruation, basal body temperature and/or the cervical mucous method. The basic idea is to regulate unprotected intercourse to the "infertile" days

However, this method has its critics...

Conception can still occur after the fertile time but the embryo is unable to implant in the uterine wall, this causes embryonic death. Luc Bovens, of the London School of economics states, "millions of rhythm method cycles per year globally depend for their success on massive embryonic death." source

who seem to base their argument on a 2006 study published in the Journal of Medical Ethics stating:


Some proponents of the pro-life movement argue against morning after pills, IUDs, and contraceptive pills on grounds of a concern for causing embryonic death. What has gone unnoticed, however, is that the pro-life line of argumentation can be extended to the rhythm method of contraception as well. Given certain plausible empirical assumptions, the rhythm method may well be responsible for a much higher number of embryonic deaths than some other contraceptive techniques.

The study seems to derive its philosophical position from data similar to the following:

It is estimated that up to half of all fertilized eggs die and are lost (aborted) spontaneously, usually before the woman knows she is pregnant. Among those women who know they are pregnant, the miscarriage rate is about 15-20%. source

I of course realize that one study is not definitive on this topic, but so far the 2006 publication is all I can find on it, which leaves me asking some questions....

Have the results showing that the rhythm method is likely to cause more embryonic death (even if it is not likely to be noticed) than other birth control methods been repeated?

If a such a large majority of the miscarriages/spontaneous abortion are unknown or unnoticed, can these findings be considered accurate?

  • I enjoyed this question, and the idea behind it.
    – user3344
    Commented Sep 13, 2011 at 13:37
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    To quote an uber-old joke, "There's a term for people practicing 'rhythm' method of birth control. They are called 'parents'".
    – user5341
    Commented Sep 13, 2011 at 19:41
  • Trivial answer: Barrier methods (e.g. condoms) prevent fertilization, thus no embryos, thus no spontaneous abortion thereof. Any positive integer > 0, QED. However, I feel that's not quite what you are asking about. Commented Sep 14, 2011 at 7:50
  • @Piskvor: Just FWIW, all positive integers are greater than 0 (that's pretty much the definition of "positive").
    – user2046
    Commented Sep 14, 2011 at 16:32
  • @Jerry Coffin: Exactly :) Commented Sep 14, 2011 at 16:34

1 Answer 1


TL;DR Answer: No firm studies have been done that I could find, however: Given the general ineffectiveness of the rhythm method, and the rate of spontaneous abortions, it follows that more fertilized eggs "die" as a result than if an egg was not ever allowed to become fertilized through other barrier or pharmacological methods.

Long Answer with citations:

According to the National Institute of Health (same citation you had now that I look at the link in your question. Other citations: University of Maryland, University of Pittsburgh, Standford University, University of Rochester with WHO data I'll come back to some of these), amongst women who know they are pregnant, the rate is 15-20%. That's the most reliable information we can get, because of the knowledge factor. Otherwise the NIH estimates a 50% spontaneous abortion rate.

Most miscarriages are caused by chromosome problems that make it impossible for the baby to develop. Usually, these problems are unrelated to the mother or father's genes.

Other possible causes for miscarriage include:

  • Drug and alcohol abuse

  • Exposure to environmental toxins

  • Hormone problems

  • Infection

  • Obesity

  • Physical problems with the mother's reproductive organs

  • Problem with the body's immune response

  • Serious body-wide ( systemic) diseases in the mother (such as uncontrolled diabetes)

  • Smoking

It is estimated that up to half of all fertilized eggs die and are lost (aborted) spontaneously, usually before the woman knows she is pregnant. Among those women who know they are pregnant, the miscarriage rate is about 15-20%. Most miscarriages occur during the first 7 weeks of pregnancy. The rate of miscarriage drops after the baby's heart beat is detected.

Given that the rate is 15-20% for those who know, then it can be logically concluded that some women will never know they are pregnant (or have a fertilized egg) and will have a spontaneous abortion (miscarriage).

Now, back to the links I provided earlier that were not NIH:

University of Rochester:

Approximately 10 to 20 percent of clinically recognized pregnancies under 20 weeks of gestation will undergo spontaneous abortion

Stanford University: A paper from 1975 entitled Estimation of Spontaneous Abortion Rates, shows a rate of about 12-26%.

University of Pittsburgh says:

Approximately one in four pregnant women experience one or more miscarriages, making spontaneous abortion the most common pregnancy complication, and of public health importance.

Again, the biggest weakness in all these studies is that they only account for the number of women who knew they were pregnant. The only study I found was from the University of Michigan Medical School that said:

The incidence of clinically unrecognized loss is approximately 20%.

I did find some sites that claimed: "some research has indicated that 70% of fertilized eggs may never go on to become a full-term pregnancy" however, none of them were referenced to a medical study. With the understanding that it's hard to collect data when someone doesn't even know that they are indeed technically pregnant.

Okay, with all this lead in material about how many actual pregnancies actually end in a miscarriage, we need to look at the effectivity of the Rhythm Method. First of all, the better term for this method is "Fertility Awareness" and here the University of Illinois gives some data (note the highlighted portion by me).

Method: Fertility Awareness

Effectiveness: 75 - 97%

STD Protection: No

Additional Information:

Utilizes daily body temperature charts, sometimes combined with daily cervical mucus checks to predict ovulation which indicates risk of pregnancy.

Requires instruction, high motivation and diligent record-keeping of fertility indicators.

Increases awareness of changes in menstrual cycle.

Requires use of back-up method or abstinence from intercourse during fertile part of cycle.

Can be an "all natural" method.

Stress, illness or vaginal infection can affect fertility indicators, which can result in pregnancy.

Not the same as "Rhythm Method" which is not a reliable method of birth control.

From the University of Utah medical library, they have information about using fertility awareness. Again, not the highlight I made:

This is also known as "natural family planning" or periodic abstinence. These include (1) the rhythm or calendar method, (2) the basal body temperatures (BBT) method, (3) the mucus method, ovulation or Billings method, and (4) the symptothermal method. They all require couple education and motivation. It is key to remember here that ovulation occurs 14 days +/- two days before menstruation. The follicular phase of the menstrual cycle is the one that varies in length, not the luteal phase. Therefore, a woman can look back and say when she ovulated, but not as easily ahead to predict the time of the next ovulation.

The rhythm method can only be used effectively in a woman with regular cycles. It is based on three assumptions: (1) The ovum can be fertilized for about 24 hours after ovulation. (Often states as up to 72 hours, but this would be a very rare exception.) (2) Spermatozoa retain their fertilizing ability for only about 48 hours. (3) Ovulation occurs 12 to 16 days prior to the onset of menses. After charting cycles for several months, a woman can establish her fertile period by subtracting 18 days from the length of her shortest cycle and 11 days from the longest cycle. The couple must then abstain from intercourse during this time. Another option for some couples is to use a barrier method during the fertile period. An example is as follows:

      A woman's menses start every 26 to 30 days.
       26 - 18 = 8. 30 - 11 = 19.
       The fertile interval is from day 8 to day 19 of the cycle.
       (Day #1 is always the first day of bleeding.)

Newer developments which have improved the effectiveness and acceptability of fertility awareness family planning involve using a basal body temperature daily to determine the end of the fertile period, checking cervical mucus for clear copious, slippery status (Spinnbarkeit) to establish the onset and end of fertility, and using a combination of all of the above (symptothermal method).

Failure rates of fertility awareness depend on the motivation and understanding of the participants. They vary from 11% to 47% and are most commonly a result of conscious deviation from the rules of the method. Most accidental (method-failure) pregnancies occur as a result of intercourse prior to ovulation.

Bottom Line: Again, the hard and fast research that has done a strict comparison on spontaneous abortions, pregnancy rates, and the method of birth control have not been fully correlated, or studied in depth. However, given the much higher probability of an egg getting fertilized using the rhythm method, and the fact that 50% or more of all fertilized eggs do not lead to a pregnancy, it follows that more fertilized eggs "die" as a result than if an egg was not ever allowed to become fertilized through other barrier or pharmacological methods. (i.e. No sperm are allowed to contact an egg with barrier methods, and no ova are released with pharmacological methods, totally preventing any type of spontaneous abortion.)

  • 3
    And a bit of a snarky blog post about this: larianlequella.blogspot.com/2009/10/god-loves-abortions.shtml Commented Feb 24, 2012 at 21:14
  • 1
    I can't see how this answers the question. It shows that there is a high rate of spontaneous abortions in the straight-forward (!) case of no contraception, but not whether the rhythm method has more than other popular contraception methods.
    – Oddthinking
    Commented Feb 25, 2012 at 1:41
  • 4
    By using the rhythm method, there is a higher probability for pregnancy overall, and thus leading to the known and unknown miscarriages. The only correlation that can be drawn here is the effectivity of the rhythm method I suppose (something I will add in). Commented Feb 25, 2012 at 3:58
  • 4
    Well, much like any loony lobby, they are lying... Barrier and pharmacological methods totally prevent ANY fertilization by preventing sperm from reaching the egg, or preventing any ovulation. References added on that as well. Commented Feb 25, 2012 at 11:02
  • 1
    @LarianLeQuella : I think it's quite reasonable to surmise that the questions asks for a prove/disprove of a correlation, which would amount to measure a percentage of miscarriages per conceptions. Saying "rhythm method => more conceptions => more miscarriages" seems like a poor joke.
    – leonbloy
    Commented Oct 7, 2013 at 20:28

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