DPO Refers to Days After Actual Ovulation, Not LH Peak
DPO (days post-ovulation) refers to the number of days after actual ovulation occurs, not the days after the LH peak. This distinction is clinically important because ovulation occurs approximately 24-36 hours after the LH surge begins and 8-20 hours after the LH peak itself 1.
Timing Relationship Between LH Peak and Ovulation
The temporal relationship between the LH surge and ovulation has been well-established through direct visualization studies:
- Ovulation occurs approximately 16.5 hours after the LH peak (95% CI: 9.5 to 23.0 hours), based on laparoscopic confirmation studies 2
- The median interval from LH peak to ovulation is 10 ± 5 hours in prospective studies with direct follicular aspiration 3
- Ovulation typically occurs 28-36 hours after the beginning of the LH rise, or 8-20 hours after the LH peak 1
Clinical Implications for Counting DPO
When calculating DPO for clinical purposes (such as timing progesterone testing, interpreting pregnancy tests, or dating early pregnancy):
- Day 0 (0 DPO) is the actual day of ovulation, not the day of LH surge detection 1, 2
- If using LH surge detection, subtract approximately 1 day to estimate the actual ovulation day, as the LH peak precedes ovulation by roughly 16-24 hours 3, 2
- In fertility treatment cycles with hCG trigger, ovulation occurs approximately 36-40 hours post-injection, so 0 DPO would be approximately 1.5 days after the trigger 4
Why This Distinction Matters Clinically
The difference between LH peak and actual ovulation has several practical implications:
- For optimal conception timing, intercourse should occur within the window from LH surge detection through 24 hours post-ovulation 1
- For progesterone testing, the standard "7 DPO" progesterone should be drawn 7 days after actual ovulation, not 7 days after LH surge detection 3
- For early pregnancy dating, gestational age calculations use the first day of the last menstrual period, which is approximately 14 days before ovulation in a 28-day cycle 5
Common Pitfall to Avoid
Do not confuse "days past LH surge" with DPO. The LH surge is a predictor of impending ovulation, not ovulation itself. Women using LH predictor kits should understand that a positive test indicates ovulation will occur in the next 24-36 hours, not that it has already occurred 1, 2. This timing difference can lead to errors in interpreting early pregnancy test results or luteal phase adequacy if the distinction is not maintained.