Calendar Method for Pregnancy: How to Use It

When you’re trying to conceive, you and your partner may want to boost your chances by tracking your fertility throughout the month. You might want to consider the old and popular method of family planning – the calendar method. Also known as the rhythm method, this could help give you some insight into when you’re most likely to conceive. Let’s run through some of the benefits and drawbacks of the calendar method for family planning.

The calendar method: What is it?


First, a quick word of background. A woman is only able to get pregnant during a certain window of time of about 5 days in a given menstrual cycle. The rhythm method is what teaches you how to recognize when you’re at peak fertility, and track the fertile window of your cycle.

Once you understand the principles behind it, it can help you work out the best time to have unprotected sex when you’re trying to conceive.

Rhythm method basics: How does it work?

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The rhythm methods run on the same principle whether you’re using it as a form of birth control, or trying to conceive: it shows the days when you’re most likely to get pregnant in relation to the day when you ovulate.

When an ovary releases a fertile egg cell at ovulation, it needs to be fertilized within several hours. On the other hand, a sperm cell can survive to a maximum of about 5 days in the female reproductive tract. That leaves a window of time of about a little bit more than 5 days per cycle when it’s possible to conceive: in the lead-up to ovulation and immediate afterward.

Your body gives off a number of signs as your hormone balance changes approaching ovulation. It’s possible to track your fertile window by charting your basal body temperature (BBT) or monitoring changes in your cervical mucus. On the other hand, all you need to start the rhythm method is a calendar, a pen, and a commitment to record-keeping.

How to use the calendar method


To use the rhythm method accurately, you will need to record the length of every menstrual cycle for at least 8 cycles. If you can do this for a full 12 cycles, even better. Count the total number of days in each cycle – that is, from Day 1 of your menstrual period through to Day 1 of the next period. You may want to circle days on a calendar, or use a smartphone app to help you keep track.

Pretty soon, you’ll end up with a record that looks a little like this example:

  • January – February: 27 days
  • February – March: 28 days
  • March – April: 30 days
  • April – May: 29 days
  • May – June: 29 days
  • June – July: 31 days
  • July – August: 28 days
  • August – September: 31 days
    …and so on.

Once you’ve got a good set of monthly menstrual data, you can start to predict when the fertile window might occur in your cycle:

Step 1

Identify the number of days in your shortest and longest menstrual cycles. In the example chart above, the shortest was 27 days, and your longest was 31 days.

Step 2

Subtract 18 from the number of days in your shortest cycle. This number is your first fertile day. For example, if your shortest cycle was 27 days long, then the first fertile day is Day 9.

Step 3

Subtract 11 from the number of days in your longest cycle. This is your last fertile day. For example, if you longest cycle was 31 days, then the last fertile day is Day 20.

If you want to get pregnant, then you should aim to have sex regularly in the potentially fertile period between Day 9 and Day 20.

Is the rhythm method right for us?

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Following the calendar method can show you the best times to have unprotected sexual intercourse when you’re trying to conceive. As long as you keep accurate records, the rhythm method could be a great addition to your pre-conception skill set!

However, calendar methods aren’t for everyone. You may not be suited to the rhythm method if:

  • You recently gave birth
  • You’re breastfeeding
  • You’re approaching menopause
  • You recently stopped using hormonal birth control
  • Your menstrual cycles are irregular
  • All your menstrual cycles are shorter than 27

The rhythm method can be a great place to start in terms of getting acquainted with your body’s natural fertility cycle. However, it doesn’t allow you to pinpoint the exact time frame of ovulation: just the phase during which it might occur. Other factors like stress or illness could also throw off your ovulation schedule.

To be more effective, it’s best to combine the rhythm method with at least one other way of identifying ovulation – particularly if the length of your cycles isn’t always the same.

Get to know your fertility cycle with the calendar method

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If a healthy and fertile woman at age 30 decides she’d like to start trying to get pregnant, she’s got about a 20% chance of a pregnancy in any given cycle.1However, if you don’t have sex during the fertile window, you might end up missing out.

Natural family planning techniques like the calendar method can help you take charge of your fertility patterns, particularly when you combine them. Unfortunately, there’s no magic formula for getting pregnant on any given try, but knowing when you’re most likely fertile can boost your chances of pregnancy success.