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If I have 26 to 28 day cycles, when am I likely to become pregnant? #833/8
I have a 26-day cycle (sometimes 27 days), if I had the first day of my period at August 4th, then I had unprotected sex at dawn of August 10th and my expected ovulation date is August 16 (the 14th day before my next menstruation), how likely am I to get pregnant?

You have 26 to 28 day cycles.  This is a very regular pattern.  Using what is called the Standard Days approach, your potentially fertile days are from day 8 to day 19.  These are the days on which to have intercourse if trying to become pregnant and to avoid intercourse if wanting NOT to become pregnant.

Standard Days Method. The first-year probability of pregnancy for women using the Standard Days Method is about 5% if the method is used correctly. When including correct and incorrect use in efficacy trials, as well as data from other field studies, the probability of pregnancy is about 12%.[i][i] Providers who taught the women participating in the efficacy study how to use the Standard Days Method received 1 to 2 days of training in the method and had no prior experience with it. When the Standard Days Method is offered by experienced providers, efficacy may improve. 

You used the Standard Days Method correctly.  This method has a failure rate of 5% in an entire year.  If you have only one act of intercourse, your risk of pregnancy would be well under 1%.



Here is a lot more information from the 19th edition of Contraceptive Technology:


Fertility Awareness Based methods of family planning use one or more indicators to identify the beginning and end of the fertile time during the menstrual cycle. These methods are effective when they are used correctly. However, with incorrect use, unpro­tected intercourse takes place precisely when the woman is potentially fertile.

In most cycles, ovulation occurs on or very near the middle of the cycle.  The fertile window of the menstrual cycle lasts for only about 6 days (the 5 days preceding ovulation and the day of ovulation), related to the lifespan of the gametes. Even though ovulation does not occur on the same day each cycle, in cycles that range between 26 and 32 days long (approximately 78% of cycles) the fertile window is highly likely to fall within cycle days 8 to 19.  In shorter or longer cycles, the fertile window shifts accordingly.

Two FAB methods, the Standard Days Method and the Calendar Rhythm Method, involve counting the days in the menstrual cycle to identify the fertile days. The Standard Days Method requires only that the woman know the day of her menstrual cycle in order to consider herself potentially fertile on days 8 through 19. Prior to starting the Calendar Rhythm Method, the woman must have recorded the length of her previous 6 to 12 menstrual cycles in order to identify the longest and shortest of these cycles. Once these are collected, calculations are performed to identify the probable days of fertility during the current cycle.  While survey results show that, in many countries, a significant number of couples state that they are using the “rhythm” or “calendar rhythm” method; most have little understanding of its proper use and simply abstain from intercourse on a few days of the woman’s cycle when they believe, often erroneously, that the woman is most likely to become preg­nant. It appears that “calendar rhythm” has become a generic term for “occasional abstinence.”

Other FAB methods, such as the Two-Day Method, the Billings Ovulation Method and the Symptother­mal Method, involve actual observation of fertile signs such as presence or absence of secretions, changes in characteristics of cervical secretions, or changes in basal body temperature. Changes in these signs are caused by fluctuations in circulating hormone levels during the cycle. Women who use these methods identify the start of the fertile time by observing cervical secretions. To identify the end of the fertile time, women can observe their cervical secretions as well as monitor the change in their basal body temperature.

by:  Victoria H. Jennings, PhD,

Marcos Arevalo, MD, MPH

Her email reply on 8-31: “Thank you for the reply - I understand you must be very busy and I appreciate the help. At my follow-up visit (now 2 weeks ago) they checked the placement of Mirena via examination and ultrasound and said it was exactly where it was supposed to be and there were no signs of problems. However, I did have additional large cysts on my left ovary now that did not show up on the ultrasound two weeks ago. He said they could be causing my intermittent pain and prescribed me Anaprox and Darvocet, and told me to come back if the pain did not stop with another menstrual cycle. I'm estimating that will be around mid-September, but am not sure as I'm always irregular. Since then, the bleeding has continued everyday and the pain averages every other day, some still severe, but the Anaprox handles most of it.  I guess after the fact I'm wondering - if I had the cysts before the IUD, why do only have the regular pain after the insertion? And is Mirena only going make the cysts worse? Should I be concerned that it is something else? Any suggestions on alternative solutions, as the medicine is wearing on my stomach a little? And are the cysts a sign that I am still ovulating or trying to, and should use back-up contraception?”

”I'm sorry my last post was so verbose - side effect of my profession. Thank you again!”

My response on 8-31: Generally cysts regress when they occur in women using Mirena IUDs.  If anything, cysts are an indication that you are NOT ovulating (protecting you from pregnancy).


Key Words:  cycle, period, unprotected sex, ovulation, menstruation, pregnant, Standard Days approach, pattern, fertile days, intercourse, Contraceptive Technology



Jennings VH, Arevalo M. Fertility awareness-based methods IN Hatcher, RA, Trussell J, Nelson AL, Cates Jr W. et al Contraceptive Technology 19th edition: pages 343-344. Ardent Media Inc. 2008

Posted 8-27-2008, Updated 8-31-2008, Updated 9-8-2008, Updated 9-11-2008 

Robert A. Hatcher MD, MPH
Emeritus Professor of Gynecology and Obstetrics
Emory University School of Medicine
Atlanta, GA

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