The day of your definitive positive reading is the day of the biggest increase in your LH.
This "LH surge" triggers ovulation and predicts it is likely to occur 24 to 36 hours later.
The accuracy of estimating your date of conception depends on several factors about your menstrual cycle and ovulation, as well as facts about the egg and the sperm.
If your cycle lengths vary a lot, your results might not be accurate.
The calculator assumes ovulation occurs 14 days before the date of an expected period as explained in the section on 'Calculation by Menstrual Cycle Length' above.
It is important to note doctors don't use the date of conception to date a pregnancy.
Instead, they date a pregnancy and estimate a due date from the date of the last menstrual period or by an early pregnancy ultrasound.
An ovulation predictor kit (OPK or LH kit) reliably predicts the window of ovulation, according to a study published in 2000 in Human Reproduction.