Fertility Awareness Methods are a collection of practices that help a woman know which days of the month she is most likely to get pregnant. A woman can learn when ovulation is coming by observing her own body and and charting physical changes. She can then use this information to avoid or encourage pregnancy. The most effective way to discover YOUR fertile time is to practice more than one techniques simultaneously. Several are explained on this website under "Knowing About Your Fertility"

Nobabynow's Alerter System uses Calendar Charting as its base technique

Fertility Awareness relies upon the following assumptions:

  • An egg (ovum) can live inside a woman’s body for 12-24 hours. However, in calculating the fertile time we use 48 hours in case more than one egg is released.
  • Sperm can live in a woman’s body up to 5 days after intercourse, though more often 2 days. Pregnancy is most likely if intercourse occurs anywhere from 3 days before ovulation until 2-3 days after ovulation.
  • Since the exact time of ovulation cannot be predicted, we add 2 to 3 days to the beginning and end.

A woman’s fertile time (“unsafe days” if she wants to prevent pregnancy) is thus about one-third of her cycle.

Calendar Charting

With Calendar Charting a woman uses past menstrual cycles as a guide. She calculates the average number of days in her cycle, and estimates future fertile times. When you know the shortest and longest cycles over several months, you can use a formula to determine an estimate of your fertile time.

Start by keeping a written record of your menstrual cycle for 8-12 consecutive months. Count the first day of menstruation (the first of bleeding) as "Day 1" of your cycle.

Pick your longest and shortest cycles. Subtract 18 from the length of your shortest cycle and note this as the first fertile day. Subtract 11 from the length of your longest cycle and this is the last fertile day.

Example: Count the number of days between Day 1 of one period and Day 1 of the next period.

29 days
26 days
(shortest cycle)
30 days
June 31 days (longest cycle)
July 30 days

In this example, the shortest cycle was 26 days and the longest was 31 days. Subtract 18 from 26 to get the first unsafe day, that is Day 8. Subtract 11 from 31 to get Day 20 as the last unsafe day. Therefore, Days 8-20 are the fertile time.

Once you have a record of your cycles, the following table will help you determine your fertile unsafe days.

22 days
Shortest Period First Unsafe Day Longest Period Last Unsafe Day
21 days 3rd day 21 days 10th day
4th day 22 days 11th day
23 days 5th day 23 days 12th day
24 days 6th day 24 days 13th day
25 days 7th day 25 days 14th day
26 days 8th day 26 days 15th day
27 days 9th day 27 days 16th day
28 days 10th day 28 days 17th day
29 days 11th day 29 days 18th day
30 days 12th day 30 days 19th day
31 days 13th day 31 days 20th day
32 days 15th day 32 days 21st day
33 days 15th day 33 days 22nd day
34 days 16th day 34 days 23rd day
35 days 17th day 35 days 24th day
36 days 18th day 36 days 25th day

Using the above chart for another example, if a woman's menstrual record shows her shortest cycle was 25 days and her longest was 30 days, her unsafe fertile days are Day 7 up to and including Day 19, counting from Day 1 as the first day of her period.

Each month add the number of days between periods to the chart and re-calculate your predictions of fertile times. As your chart grows, cross off the oldest cycles and only consider the past 12 months.




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    The results of the calculator may vary if your periods are irregular. Do not use it to predict your ovulation dates if your periods are irregular.