LH, or luteinizing hormone, signals to the ovaries that it is time to release an egg by surging to approximately four times its normal levels directly preceding ovulation. Ovulation predictor kits are generally used by couples hoping to become pregnant to test for an LH surge in the woman, thus pinpointing when ovulation will occur to better time intercourse. While how long after an LH surge ovulation occurs can vary from woman to woman and from cycle to cycle, generally ovulation occurs 12 to 48 hours after a detected LH surge, according to an Ovulation Detection fact sheet from the American Society for Reproductive Medicine.
For couples trying to conceive figuring out how long after an LH surge will ovulation occur can be the determining factor in whether they achieve pregnancy that cycle or have to wait and try again. LH is involved in the regulation of a woman's menstrual cycle and the timing of ovulation. For women with a typical 28 day menstrual cycle ovulation generally occurs midway through the cycle on day 14, according to the American Society for Reproductive Medicine.
Many women never monitor their LH surge, but for those with difficulty getting pregnant or trouble identifying when they ovulate, detecting an LH surge can be helpful. An LH surge is typically proven through the use of at home ovulation predictor kits using a urine specimen. On average, women ovulate 36 hours after an LH surge. This information helps couples time their intercourse for a greater chance of getting pregnant.
Couples can also have a doctor perform a blood test to determine if an LH surge has occurred, but this is typically only necessary for couples with suspected infertility problems. Ovulation predictor kits are widely available at drugstores and are similar in design to home pregnancy tests. To catch the LH surge women should test daily beginning on day 11 of a 28-day cycle. Tests should be taken at the same time each day (afternoon is recommended) to ensure women do not miss the LH surge.
For the best chances of conceiving couples should have intercourse on the day of the LH surge and for at least the next two days, according to the Fertility Plus Ovulation Predictor Kit FAQ. Timing intercourse on these days will help to ensure that sperm are present in the Fallopian tube when the egg is released, so fertilisation can occur.
Determining how long after an LH surge you ovulate and timing intercourse accordingly does not ensure a pregnancy. Even under ideal conditions and perfect timing, it may still take couples several cycles to achieve pregnancy. For young, otherwise healthy couples, infertility problems are not suspected until 12 months of trying to conceive, with good timing, without a resulting pregnancy.