Irregular Cycle: How to Know When You Ovulate, and When to See a Doctor

One month, your period arrives after 26 days. The next, you wait 41, with a negative pregnancy test in between. When you have an irregular cycle and a baby plan, every delay becomes a puzzle, and the apps announcing 'ovulation tomorrow' reassure nobody any more.

An irregular cycle does not stop you getting pregnant. It simply makes ovulation harder to predict, and sometimes deserves a search for the cause. Here are the benchmarks to help you find your way.

What is an irregular cycle?

A cycle is counted from the first day of your period to the day before the next one. According to the Quebec guide Mieux vivre (INSPQ), it generally lasts 21 to 35 days, with an average of 28 to 30. The UK's NHS speaks of irregular periods when the gap between periods is under 21 days or over 35 days, or when it changes a lot from month to month.

A few days of variation are commonplace: a 27-day cycle followed by a 31-day cycle is still regular in medical terms. When periods stop for several months, it is called amenorrhoea; the NHS advises seeing a doctor after three missed periods in a row.

The most common causes

The NHS gives a list that mixes life stages and health problems:

• Turning-point ages: the first years after puberty, then the lead-up to the menopause, generally between 45 and 55.
• A pregnancy, to be ruled out first with a test.
• Breastfeeding, which often delays the return of periods; see our article on the return of periods after birth.
• Hormonal contraception: some methods (progestogen-only pill, injection, hormonal coil) change bleeding, and the cycle can take a few months to settle after stopping, as our article on coming off the pill explains.
• Weight: rapid or major weight loss, or marked overweight.
• Stress and intense exercise: the Mayo Clinic notes that very strenuous sport can disrupt ovulation in a woman of normal weight.
• PCOS, polycystic ovary syndrome, which the NHS now calls PMOS (polyendocrine metabolic ovarian syndrome). It is a common cause of long, irregular cycles, often with acne, excess hair or weight gain. It is all explained in our article on PCOS and pregnancy.
• The thyroid, whether overactive or underactive.

Why does an irregular cycle make ovulation harder to calculate?

Because ovulation does not happen on day 14 of the cycle, but about 14 days before your next period (INSPQ). With a 28-day cycle, that falls around day 14. With a 40-day cycle, closer to day 26. And when you do not know when the next period will come, you cannot count backwards.

Tracking apps do this calculation from your past cycles. With irregular cycles, they are often wrong: the ASRM cites a study in which the most accurate app found the day of ovulation in only 21% of cases. The principle of the calculation, for a regular cycle, is set out in our guide to ovulation and fertile days.

There are also cycles without ovulation, more common when periods are far apart. The NHS lists a lack of regular ovulation among the usual causes of difficulty conceiving.

Spotting your ovulation: tools and their limits

Watching your mucus is more useful than people think: according to the ASRM, the chance of pregnancy reaches about 38% on the day the mucus is at its most stretchy.

Can you regulate your cycle?

It depends on the cause, which is why an irregular cycle is not treated 'at random'. The NHS says treatment is not always needed. When it is, it targets the cause: treating a thyroid disorder, managing PCOS, adjusting contraception.

Weight is a real lever. According to Britain's NICE, a woman with a BMI under 19 and irregular or absent periods improves her chances of pregnancy by gaining weight; conversely, when BMI is above 30 and ovulation is lacking, weight loss, ideally supported in a group with physical activity and dietary advice, improves the chances. On the plate side, our piece on diet and fertility sorts evidence from received wisdom.

Be wary, though, of herbal teas and mixtures sold to 'regulate periods' or 'cleanse the womb': NICE points out that the effect of complementary medicine on fertility has not been properly evaluated.

When should you see a doctor?

Sooner than other women, if you want a child. For a couple with no particular concern, you see a doctor after 12 months of trying, or 6 months from age 35 (see how long it takes to get pregnant). But the NHS advises mentioning irregular periods to a doctor as soon as they appear or change, and a very irregular cycle is a reason not to wait a year of trying.

In Morocco, the first port of call can be your GP, your gynaecologist or the health centre. Depending on what emerges from the questions and examination, the doctor may order further tests, such as a blood test or an ultrasound, and refer you to a specialist if needed. The tests offered to the couple are detailed in our article on infertility: when to see a doctor.