What is the luteal phase?
The luteal phase, also referred to as 'days past ovulation' or 'DPO', is the part of the cycle that starts at ovulation and ends the day before your next period. It usually lasts about 14 days and does not vary by more than a day in each person. 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. Your luteal phase must be at least 10 days long to support pregnancy.

The importance of the luteal phase
The length of the luteal phase determines the time of ovulation within your menstrual cycle. Ovulation can be delayed by a number of factors, such as stress, increased activity or medication, but the length of the luteal phase is usually constant. Taking this into account, you can calculate the 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, the ovulation will occur on day 16 of your cycle (28-12=16). Ovulation Calendar uses this formula to calculate your time of ovulation.

How do I determine the length of my luteal phase?
The only way to determine the exact length of your luteal phase is through hormone-specific blood tests.

What if I don't know the length of my luteal phase?
If you are not sure about the length of your luteal phase, you can assume it is 14 days (the average length for most women).

Information provided by yourdays.com