The calendar-based method is a fertility awareness method that uses your menstrual cycle history to estimate which days you’ll be at risk of becoming pregnant if you have unprotected sex.
This article is part of our Contraception Series!
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Note: the information below was sourced from references (1-5), unless stated otherwise.
What is it?
The calendar-based method is a fertility awareness method that uses your menstrual cycle history to estimate which days you’ll be at risk of becoming pregnant if you have unprotected sex. The idea is that you’ll be able to have sex without using other contraception on all other days. Of course, you can still have sex on fertile days, as long as you use a different contraception on those days (for example, a condom).
How does it work?
There are several variations of the calendar-based method. For all of them, you have to closely track your menstruations. Based on a regular cycle (which lasts 26 to 32 days), you’ll ovulate around two weeks after the start of your period. When an egg has been released, it can be fertilized for about 24 hours. This knowledge, in combination with the fact that sperm cells can stay alive for five to seven days, is then used to calculate when you’ll be fertile. In practice, this means you can’t have unprotected sex for about a week leading up to your ovulation until the day of ovulation itself and the day after.
While this method works in theory, many people’s cycles are not so regular that you can exactly calculate when you’ll ovulate without using any other indicators. The time between menstruation and ovulation is therefore a tricky time to have unprotected sex. After ovulation, however, you’re quite reliably non-fertile - though it's also hard to pinpoint exactly when you're ovulating.
In order to use a fertility awareness method reliably, you need to receive high-quality counselling. Ask your doctor whether they can provide this to you, or perhaps to refer you to an expert.
How effective is it?
There is not a lot of data showing how effective fertility awareness methods are. It is estimated that it’s about 95% effective with perfect use (meaning 5 people* out of 100 using this method become pregnant within a year), while this decreases to 88% to 76% effectivity with typical use (12 to 24 people* out of 100 will become pregnant within a year). Higher rates of pregnancy are more likely when you haven’t had good counseling on these methods by a medical professional.
What are possible side-effects?
There are no side-effects of a fertility awareness method.
When can't I use it?
When you don’t have a regular cycle (which you can track using your calendar or a period-tracking app), because estimating when you’re ovulating is nearly impossible in that case.
The term “irregular cycle” is typically used if your cycle lengths (from the start of menstruation to the start of your next menstruation) differ by more than seven days, they’re shorter than 26 days, or longer than 32 days. For example, if your shortest cycle was 25 days and your longest was 35 days, that’s a difference of ten days and your cycle might be considered irregular. Irregular cycles are more common in the two years after your first period, around menopause, before you’ve had three postpartum menstruations, and before you’ve had one menstruation after abortion. Use of this method is therefore not recommended in these periods.
You also shouldn’t use this method if you have genital bleeding for an unknown reason, or if you use any drugs that could affect the regularity of your cycle, your hormones, or fertility signs (for example, lithium, certain medications for depression and anxiety, several anti-inflammatory drugs, and more).
Lastly, you shouldn’t use a fertility awareness method when you have a condition that puts you in great danger if you do become pregnant. These methods are not reliable enough to protect you in this case.
Does it work immediately?
No. You have to track your cycles for at least six months to gain enough insight into your regularity to correctly estimate when you might be fertile (6). You should also keep an eye on future menstruations to notice in time when your cycle lengths start changing.
What happens to my fertility if I stop using it?
Since nothing in your body changes when using this method, nothing’s different when you stop using this method. You have a higher risk of pregnancy if you’re not tracking your fertile days, because you won’t know when your fertile days will be.
Does it protect against sexually transmitted diseases (STDs)?
No, fertility awareness methods do not protect against STDs. If you’re having sex with someone new or untested, use a barrier method as well.
*People, here, means anyone who is able to become pregnant, including girls, women, and non-binary people and transgender men who still have their uterus, vagina, and ovaries.
Are you curious about other methods to protect yourself from an unwanted pregnancy? Read about other birth control options here!
This article is pending medical review.
Written by Juliëtte Gossens
Reviewed by Sophie Oppelt and Selina Voßen
Edited by Juliëtte Gossens
McFarlane I (ed.). Seeing the unseen: The case for action in the neglected crisis of unintended pregnancy. United Nations Population Fund. 2022. Available from: https://www.unfpa.org/sites/default/files/pub-pdf/EN_SWP22%20report_0.pdf
Hacker NF, Gambone JC, Hobel CJ (eds.). Hacker & Moore’s Essentials of Obstetrics & Gynecology. 6th ed. Philadelphia: Elsevier; 2016.
Hoffman BL, Schorge JO, Halvorson LM, Hamid CA, Corton MM, Schaffer JI (eds.). William’s Gynecology. 4th ed. New York: McGraw-Hill Education; 2020.
Centers for Disease Control and Prevention. The United States Medical Eligibility Criteria for Contraceptive Use, 2016 (US MEC). Available from: https://www.cdc.gov/reproductivehealth/contraception/mmwr/mec/summary.html
Centers for Disease Control and Prevention. 2016 U.S. Selected Practice Recommendations for Contraceptive Use (U.S. SPR). Available from: https://www.cdc.gov/reproductivehealth/contraception/mmwr/spr/summary.html
Fertility UK. Avoiding Pregnancy. Available from: https://www.fertilityuk.org/new-page-1 [Accessed September 21st, 2022]
Please note: the information we provide to you here is for educational purposes only. If you’re experiencing any discomfort or have any complaints or questions about your health, please contact your doctor or other relevant health professional. We don’t provide medical advice.