The luteal phase is a period starting on the first day after ovulation and lasting until the end of the cycle (the day before the next menstruation). At OvuFriend, after ovulation has been detected, your DPOs (Days Past Ovulation) start to add up on the chart. The sum of these days equals the length of your luteal phase. Luteal phase length varies from between 10 to 16 days, with an average of 14 days. Its length is rather constant and does not vary from cycle to cycle. By knowing the length of your luteal phase, you can accurately calculate when after ovulation you should expect your next menstruation, and when it's best to take a pregnancy test.