Ovulation predictor kits and the LH surge they detect

Ovulation predictor kits, often called ovulation tests or OPKs, measure luteinizing hormone (LH) in urine. The pituitary gland makes a small amount of LH throughout the cycle and a much larger amount just before ovulation, and the test turns positive when it detects that rise, known as the LH surge (US Food and Drug Administration, 2018).

A positive ovulation test means ovulation is likely within about two days

The FDA places the LH surge about one to one and a half days before ovulation. The American Society for Reproductive Medicine (ASRM) gives a slightly wider interval in its 2022 committee opinion on optimizing natural fertility: ovulation may occur at any time within the two days after the surge. The fertile window is the five days before ovulation and the day of ovulation itself (Wilcox, Weinberg and Baird, 1995), so a positive result falls near the end of the window, on the days with the highest chance of conception. The page on the most fertile days sets out those figures.

Some kits also measure estrone-3-glucuronide (E3G), a breakdown product of estrogen that builds up in urine as ovulation approaches. Kits that read both hormones can also mark days of high, but not peak, fertility, in addition to the surge itself (ASRM, 2022).

Testing has to begin before the surge, so the first test day depends on cycle length

A surge that comes only a day or two before ovulation is easy to miss if testing starts late. Kit instructions set the first test day from the usual length of the cycle, and the FDA warns that testing begun at the wrong point in the cycle can miss the surge altogether. The ovulation calculator on this site estimates the day of ovulation from cycle length and luteal phase length, which gives a starting point. It is an estimate from averages, and the true day varies from cycle to cycle.

ASRM's 2021 committee opinion on fertility evaluation notes that urine results match the peak of LH in the blood well, particularly with midday or evening urine samples. Products differ on the time of day they specify, and the instructions for the specific test apply. A daily record of results across the cycle, sometimes called an ovulation test chart, shows the day the surge appeared and whether that day moves from one cycle to the next.

Ovulation tests detect the surge reliably but cannot confirm that an egg was released

The FDA reports that home tests detect LH, and E3G where they measure it, about 9 times in 10 when the instructions are followed carefully. In a study of 101 women attending for infertility, the urine LH surge came before ultrasound evidence of ovulation in every cycle and agreed with the ultrasound finding in 98 of the 101 (Guermandi and colleagues, Obstetrics & Gynecology, 2001). The same study found that the time from the surge to ovulation varied widely.

A positive test shows that the surge happened. It does not show that ovulation followed. ASRM (2022) reports false-positive results in about 7 in 100 cycles, and its 2021 opinion adds that accuracy, ease of use and reliability vary between products. Confirming ovulation needs a measure taken afterwards, such as a sustained rise in basal body temperature, or a blood progesterone test arranged by a clinician about a week before the next period is due, which ASRM (2021) describes as a reliable measure of ovulation when it is correctly timed. A confirmed ovulation marks the start of the luteal phase, the steadier second half of the cycle.

Timing sex with an ovulation test may raise the chance of pregnancy, on weak evidence

ASRM (2022) cites a randomized trial in which ovulation detection devices shortened the time to pregnancy, and notes that using fertility-tracking methods to time intercourse is associated with a higher probability of conceiving in an ovulatory cycle. A Cochrane review was more cautious (Manders and colleagues, 2015). Across four randomized trials with 1,387 couples, timed intercourse was associated with more pregnancies: where 13 in 100 would conceive without ovulation prediction, between 14 and 23 in 100 would conceive with it. The reviewers rated this evidence as very low in quality, counted pregnancies not yet confirmed by ultrasound, and found too little data to judge the effect on live births.

ASRM (2022) also advises that a test be used to guide the timing of frequent intercourse rather than to replace it. It finds the chance of pregnancy highest with intercourse every one to two days in the fertile window, and more frequent intercourse does not lower it.

Irregular cycles and PMOS make the results harder to interpret

With irregular cycles the first test day is harder to choose, and a long cycle can use many tests. People with polyendocrine metabolic ovarian syndrome (PMOS), the condition formerly called polycystic ovary syndrome (PCOS), can have raised LH levels throughout the cycle, which can give positive results when no surge is under way (ASRM, 2021). Patient guidance from the American College of Obstetricians and Gynecologists (ACOG), in its article on trying to get pregnant, goes further: with highly irregular periods, predicting ovulation at home is unlikely to be worth the time or money, and such periods can be a sign that ovulation is not happening (ACOG, 2023). The pages on ovulation and irregular periods, irregular periods and PCOS and pregnancy cover this in more detail.

UK guidance puts less weight on timing than US guidance

US guidance describes sex timed to the fertile window. Besides ASRM's interval of every one to two days in the window, ACOG's patient guidance (2023) describes tools such as ovulation kits and apps as useful for some couples, particularly those who have sex only once or twice a month, and not necessary for most. The NHS frames it differently. Its guidance on trying to get pregnant (2026) states that people aged 39 or under who have sex without contraception every 2 to 3 days are likely to conceive within a year, and its guidance on natural family planning (2024) notes that none of the apps or fertility monitoring devices on the market is officially recommended by the NHS.

The FDA (2018) states that ovulation tests are not reliable as a way to prevent pregnancy.