What’s the difference between the Fertility Awareness Method and the Rhythm Method?

If the answer to this question were better understood, FAM would be more well-known and respected – especially by the medical community.

As a natural form of birth control, FAM is continually lumped in with the Rhythm Method, even though Rhythm has been essentially defunct and discredited for over 20 years. This is especially frustrating considering FAM’s impressively high effectiveness rate, which is on par with that of the birth control pill.

So let’s settle this thing once and for all. What is the difference between FAM and the Rhythm Method? Is it possible for systems of natural birth control to be fundamentally different, or are they all the same? What could makes FAM highly effective and Rhythm justifiably laughable?

First, let’s define our terms.

For a detailed description of the Fertility Awareness Method, read our What is FAM? page.

How does the Rhythm Method work?

From Wikipedia:

To find the estimated length of the pre-ovulatory infertile phase, nineteen is subtracted from the length of the woman’s shortest cycle. To find the estimated start of the post-ovulatory infertile phase, ten is subtracted from the length of the woman’s longest cycle. A woman whose menstrual cycles ranged in length from 30 to 36 days would be estimated to be infertile for the first 11 days of her cycle (30-19=11), to be fertile on days 12-25, and to resume infertility on day 26 (36-10=26).

Another variation of Rhythm has you count back 14 days from your period, assume that this was the day you ovulated in your previous cycle, and assume that this is the cycle day when you will ovulate again in the current cycle. (There’s a lot of assuming going on here.) You then avoid unprotected intercourse around the time when you think you will ovulate.

Hopefully you can see some obvious problems with this.

So what is the difference between FAM and the Rhythm Method?

1) FAM is a symptoms-based method while Rhythm is a calendar-based method.

FAM uses biological symptoms of fertility (including basal body temperature, cervical fluid and cervical position) to assess day-to-day fertility. Scientifically proven rules are then used to interpret the symptoms and inform your choice of whether or not unprotected intercourse is safe.

Rhythm uses past cycles to predict future fertility. There is no day-to-day assessment of fertility. Rhythm hinges on future cycles proceeding exactly like past cycles. Any cycle length variation outside of a narrow range can cause Rhythm to fail.

2) The requirements of Rhythm are not realistic for most women. Because Rhythm doesn’t account for natural fluctuations in cycle length, it can require long periods of abstinence or using a barrier method.

Lifestyle factors such as stress, travel and exercise all affect cycle length. I have had cycles as short as 28 days and as long as 52 days, with the long cycles easily attributed to stress, travel or intensive exercise training. If I were to follow the requirements of the Rhythm Method, I wouldn’t be able to consider myself infertile until cycle day 42. Considering that the vast majority of my cycles are only 30-something days long, this really doesn’t make any sense.

FAM allows you to assess your fertility on a day-to-day basis. The length of previous cycles doesn’t affect your assessment of your current cycle (unless you are using a Doring-based rule, but that is another discussion). Having previous cycle history can be helpful in knowing how your cycles usually progress, but ultimately your decision of whether you are fertile or infertile is based on your current cycle and your current biological symptoms.

3) FAM also allows you to choose the rules that best fit your situation and need for efficacy. If you definitely need to avoid pregnancy, you can choose very strict rules. If you aren’t as concerned about a potential pregnancy and want to allow yourself more days of unprotected intercourse, you can choose rules that are less strict. Rhythm only has one set of rules that aren’t especially conservative.

Example: In the first new cycle after we got married, I had an unusually short cycle that was 28 days long. According to Rhythm I would have been safe through cycle day 10, but in reality I ovulated only 4 days later. Since sperm can survive for up to 5 days in ideal conditions, this would have been close enough for pregnancy.

Ultimately FAM and Rhythm are different because FAM is conservative and flexible enough to deal with cycle variations, and Rhythm is not.