The luteal phase, also referred to as "days past ovulation", "DPO", or the "postovulatory phase" is the part of the cycle that starts at ovulation and ends the day before your next period. The average length of the human luteal phase is fourteen days (2 weeks). Between ten and sixteen days is considered normal, although luteal phases of less than twelve days may make it more difficult to achieve pregnancy. While luteal phase length varies significantly from woman to woman, for the same woman the length will be fairly consisten from cycle to cycle. The luteal phase is named after the corpus luteum (Latin: "yellow body"), a structure that grows on the surface of the ovary where a mature egg was released at ovulation. The corpus luteum produces progesterone in preparing the body for pregnancy. At the onset of the luteal phase, women's body temperature increases (Basal Body Temperature) in order to provide a fertile environment for the ovum.
The length of the luteal phase can be used to help determine the time of ovulation within your menstrual cycle. Ovulation can be delayed by a number of factors, such as stress, increased activity, illness or medication, but the length of the luteal phase is usually constant. Taking this into account, you can calculate the estimated time of ovulation within your cycle by subtracting the length of your luteal phase from the length of your cycle. For example, if your cycle is 28 days long and your luteal phase is 12 days long, ovulation should occur on day 16 of your cycle (28-12=16). The Your Days' Ovulation Calendar uses this formula to calculate when you will ovulate.
You can determine the length of your luteal phase by charting your Basal Body Temperature (BBT), also known as your waking temperature. As mentioned above, your BBT rises after ovulation. By counting the number of days from your thermal shift until your period, not including the first day of menses, you will have the length of your luteal phase. Charting your BBT is actually easier than you think and anybody can do it:
1. Using a regular thermometer (that measures within 1/10th of a degree F., such as 97.4) take your daily termperature first thing upon awakening, before any other activity such as drinking water, talking on the phone, or getting up to use the bathroom.
2. Try to take your temperature about the same time every morning give or take about an hour. However you don't need to be a slave to your thermometer. If you sleep in one day and wind up taking it later, just be sure to note the time on the chart, because basal temperatures tend to creep up the later you take them.
3. Note down your temperature on your Ovulation Calendar or BBT chart.
4. Unusual events such as stress, illness, travel or moving should be noted when interpreting the temperature pattern. And temperatures taken earlier or later than usual should be noted.
5. Record and connect the temperatures with a pen to get a range.
6. When you note that your temperature has shifted and remained at a higher range you have begun your luteal phase and ovulation has occured. After ovulation, temperatures quickly rise above the range of lows that preceded it. This thermal shift is often so obvious that you'll be able to spot it simply by glancing the chart.
7. When your period starts your luteal phase has ended. Your temperature will usually go back down to the lower ranges at this point. The length of your luteal phase can be counted from the day you ovulated (the first day of your thermal shift) to the last day before your period began.
We highly recommend that you try charting your waking temperature following the above steps. This will help you to determine the length of your luteal phase.
However most women have a luteal phase that lasts 10 - 16 days - averaging at 14 days. If you are not sure about the length of your luteal phase, you may safely use the 14 day average as the default value for the purpose of ovulation calculation. The Ovulation Calendar will still give you a very good estimation of when you will be the most fertile.