Working With a Fertility Calculator


9:04 am January 11, 2012

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A fertility calculator is a neat little online tool that women can use to chart their most fertile days. This is useful for couples who are trying to get pregnant as well as for those who don’t want to conceive just yet. A fertility calculator works like this: You key in certain information about your cycle, including the date of your most recent period and the length of your periods and it figures out the most likely date for your next ovulation. However, one setback of working with a fertility calculator is you need to have a regular cycle. For those who have erratic periods, you can always do the calculating manually.

Calculating Your Fertile Days

Whether you are using a fertility calculator or just a pen and paper, it is important that you first chart your periods at least six months before you want to start predicting your next period. This is so you can establish a pattern upon which you can rely your next ovulation date on. Here is how to figure out your fertility period.

  1. Count the number of days between the start of one period and the start of the next period. From these six figures, take the shortest and longest intervals between your periods.
  2. Subtract 18 days from the shortest interval. For instance, if the shortest interval is 28 days, you come up with the 10th day from Day 1 of your period, or the 10th day of your cycle.
  3. Subtract 11 days from the longest interval. For instance, if the longest time between your periods was 32 days, you arrive at the 21st day of your cycle.

Making Sense of these Numbers

A fertility calculator won’t be able to explain to you the significance of these numbers. To make sense out of the number-crunching, you have to know that there are three phases in one menstrual cycle.

  • First Infertile Phase.This occurs from the first day of the cycle, also the first day of your period, until the earliest day from which sperm cells could stay alive. In our example, the first infertile phase starts on Day 1 and ends on Day 10.
  • Fertile Phase.This is when a woman is most likely to get pregnant. In our example, the fertile phase starts on Day 11 until 24 hours after ovulation has occurred.
  • Second Infertile Phase.The mature egg cell dies away 24 hours after being released without having been fertilized by a sperm. This is the start of the second infertile phase, which lasts until the start of the next cycle.


Not all women are lucky enough to have regular periods. These are defined by having cycles that take place regularly in 25 to 35 days. Anything intermittent, or periods that are accompanied by dramatically different amounts of bleeding, is deemed irregular. Women who have irregular periods, unfortunately, find no use for a fertility calculator and will have to rely on other methods, such as ovulation predictor kits and medicines that stimulate hyper-ovulation to get pregnant.

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