If I get a positive LH test, when should I have sex in order to conceive?

LH surge. With urine testing, the lh peak should be about 24 hours before ovulation. That said, the test can give erroneous results in almost 10 percent of cycles. The best way to use the test is to determine, over 3 cycles, on which cycle day your surge normally occurs. Time sex for a few days before the surge through one day after the surge. That should increase your odds for conception.