Calendrical method

Related Terms

Basal body temperature, calendrical method, conception, fallopian tube, family planning method, fertility, luteinizing hormone, menstrual cycle, menstruation, ovulation, pregnancy.

Background

Ovulation is the moment during a woman's menstrual cycle when the ovarian follicle releases the egg. This is the time of peak fertility for a woman. Predicting when a woman will ovulate by way of calendrical method or fertility charting is called ovulation calculation. This ovulation calculator provides approximate dates for predicting ovulation. There are several variables that can cause cycle irregularities. To ensure accuracy, ovulation calculator results may be used in conjunction with other predictive tools.
Other methods predict ovulation by detecting luteinizing hormone (LH). Just preceding ovulation, women experience an "LH surge" - a sudden, dramatic and brief rise in the level of luteinizing hormone. Ovulation tests detect this LH surge, allowing women to accurately predict when they will ovulate. A positive result on an ovulation test means that the woman will most likely become fertile over the next three days - with peak fertility at 36 hours following the LH surge. Basal body temperature, changes in cervical fluid and fertility charting have also been used to detect ovulation, as has fertility charting, which helps predict
Note: The ovulation calculator cannot be used to reliably to prevent pregnancy.

Theory / Evidence

The ovulation calculator provides approximate dates only and assumes a regular menstrual cycle. If menstrual cycles are irregular, it may be difficult to pinpoint the day of ovulation based on the calendar alone. In that case, individuals may want to monitor their basal body temperature and cervical mucus. This ovulation calculator shows the most fertile days for getting pregnant. Plans are based on the fact that, for most women, ovulation happens approximately 14 days before the next menstrual period. As ovulation approaches, the chances of getting pregnant increase. Knowing when ovulation occurs helps individuals time intercourse that is more likely to result in conception.
For women who have a 28-32 day menstrual cycle, ovulation can take place between days 11 through 21.This is just an average of days that ovulation may take place - every woman's cycle is different. A woman who has a 27-28 day cycle will most likely ovulate in the earlier part of the window. Those with a longer cycle will often ovulate later in the fertility window.
The calculator is based on the above method of calendar estimation. However, other factors may contribute to the ovulation window, including the length of the luteal phase, which is the time period beginning with the day after ovulation and running through the remainder of the menstrual cycle (ends the day before your next period). Typically, the duration of the luteal phase lasts between 10 and 16 days, and it is generally consistent from cycle to cycle, averaging for most women at 14 days. For this calculator, the luteal phase is set to 14 days. The ovulation calculator may not give accurate readings for those with cycle lengths of more than 35 days, or shorter than 21 days.

Author information

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

Bibliography

American Pregnancy Association. 8 May 2006.
Hamilton BE, Martin JA, Sutton PD. Births: Preliminary Data for 2003. National Vital Statistics Reports. 2004; 53(9)1-18.
March of Dimes. 8 May 2006.
Martin JA, Hamilton BE, Sutton PD, et al. Births: Final data for 2002. National Vital Statistics Reports. 2003; 52(10)1-114.
Maternal and Infant Health: Infant Health. Center for Disease Control and Prevention. 8 May 2006.

Charts and calculators

In order to calculate your approximate ovulation time, answer the following questions:
When was the first day of your last period?
How many days are there in your cycle?
What is the length of your luteal phase? (If known)
Next period: Date of last period plus cycle length. (This varies from person to person, but the following calculations are based on a regular period, regardless of how many days.) Example: May 1 + 28 days per cycle = May 29.
Ovulation date: Next period minus luteal phase. Example: May 29 - 14 days (luteal phase) = May 15.
Range: If unsure about the length of your luteal phase, take the expected day of your next period and subtract 10 from it. This is the end of your fertility range. Example: May 29 - 10 days = May 19.
Then repeat and subtract 16 days to find out the beginning of your fertility range. Example: May 29 - 16 days = May 13
These are the range of dates that you are most likely to be ovulating. Example: May 13 through May 19.
Note: Every woman's body is different and these calculators can only be used as predictive methods, but may not be 100% accurate and thus should not be used to reliably prevent pregnancy.