Ovulation and irregular periods when trying to conceive

The phase after ovulation is the most consistent part of the menstrual cycle, according to the American College of Obstetricians and Gynecologists (ACOG) guidance on fertility awareness-based methods (ACOG, 2019). Most of the difference between one cycle and the next therefore lies in the days before ovulation, the follicular phase, and a cycle that arrives late usually means an ovulation that came late or did not come at all.

A cycle counts as irregular when it falls outside 21 to 35 days

The NHS guidance on irregular periods (2026) defines periods as irregular when the gap between them is less than 21 days or more than 35 days, and ACOG gives the same 21-to-35-day range for a normal cycle (ACOG, 2021). The ovulation calculator on this site uses the same range and flags cycle lengths outside it; the guide to irregular periods compares the US and UK definitions and covers the causes.

Irregular cycles mostly reflect variation in the days before ovulation

ASRM puts the normal length of the phase after ovulation, the luteal phase, at 11 to 17 days, 14 on average (ASRM and SREI, luteal phase deficiency opinion, 2026). In a study of 221 women, the 16 in 100 who described their cycles as irregular ovulated later and at more variable times than the others, so their fertile days were spread more widely across the cycle. Across the whole study, ovulation came as early as day 8 and as late as day 60 (Wilcox, Dunson and Baird, BMJ, 2000). The page on the most fertile days sets out how far the fertile window moves even in regular cycles, and the guide to menstrual cycle phases shows where each phase falls.

Very long or absent cycles can mean ovulation is not happening

ASRM's 2021 committee opinion on fertility evaluation treats a history of infrequent periods (oligomenorrhea) or absent periods (amenorrhea) as enough on its own to establish anovulation, meaning cycles in which no egg is released. It reports that problems with ovulation account for up to 40 in 100 cases of infertility in women. In women with regular cycles, occasional cycles without ovulation are relatively rare, at an estimated 1 to 14 in 100 (ASRM, "Fertility evaluation of infertile women", 2021).

The NHS (2026) lists common causes of irregular periods: puberty, the approach of menopause, pregnancy, some hormonal contraception (including the progestogen-only pill, the contraceptive injection and the intrauterine system, which US sources call the hormonal IUD), losing or gaining a lot of weight, stress and anxiety, and excessive exercise. It adds that conditions such as polyendocrine metabolic ovarian syndrome (PMOS), the condition formerly called polycystic ovary syndrome (PCOS), and thyroid problems can also cause irregular periods. ASRM (2021) names PMOS among the most common causes of problems with ovulation. The guide to PCOS and pregnancy covers getting pregnant with the condition.

Calendar estimates and ovulation tests are less reliable when cycles vary

A calendar estimate projects ovulation from the length of past cycles. ASRM's 2022 committee opinion on optimizing natural fertility identifies the weakness of that approach even in general use: it assumes the timing of the fertile window is consistent, when cycles vary much more than that. With irregular cycles, the length of the last cycle says little about the next one, and the ovulation calculator's estimate becomes much less certain.

Urine LH tests still detect a surge, but the first test day is harder to choose and a long cycle can use many tests. In PMOS, LH can stay raised through the cycle, which can produce positive results without a surge (ASRM, "Fertility evaluation of infertile women", 2021). ACOG's patient 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 page on ovulation predictor kits covers how the tests work.

Cervical mucus and basal body temperature respond to the hormones of the current cycle rather than to an average, and ovulatory cycles are generally associated with a two-level temperature chart, although some women who do ovulate never show one (ASRM, "Fertility evaluation of infertile women", 2021). The NHS notes, in its guidance on natural family planning (2024), that fertility awareness methods may not suit people with irregular periods who want to avoid pregnancy.

Can you get pregnant with irregular periods? UK and US guidance favor regular sex and earlier advice

For people with irregular periods who are trying to conceive, the NHS (2026) suggests sex every 2 or 3 days throughout the cycle without contraception, and advises seeing a GP when periods are irregular and pregnancy is not happening. ASRM (2021) lists irregular cycles, cycles shorter than 25 days, and infrequent or absent periods among the conditions for which fertility testing can begin without delay, rather than after the usual 12 months of trying. ACOG's patient guidance (2023) likewise points people with highly irregular periods to an ob-gyn or a reproductive endocrinologist rather than to home prediction. Where treatment is needed, the NHS (2026) notes that it depends on the cause.