Irregular Periods and Trying to Conceive: What to Know

If your periods don’t arrive on a schedule, every part of trying to conceive is harder: you can’t count days to find the fertile window, and every late period raises a question. Irregular periods are also one of the clearest signals in fertility care, because they usually point to how often you ovulate. This guide explains what counts as irregular, the main causes, how to track ovulation, and why you can get help now rather than after a year.

What counts as an irregular period?

The 2023 International PCOS Guideline gives the definitions most clinicians use. From three years after your first period until perimenopause, cycles are irregular if they are shorter than 21 days, longer than 35 days, or fewer than 8 a year.¹ Any single cycle longer than 90 days is irregular from one year after the first period.¹

Definitions from the 2023 International PCOS Guideline.¹

Why do irregular periods affect fertility?

ASRM says ovulatory dysfunction is found in about 15% of infertile couples and accounts for up to 40% of infertility in women, and that it usually shows up as infrequent or absent periods.² When cycles are long or unpredictable, ovulation is happening less often, later, or not at all, so there are fewer chances to conceive each year. Ovulation problems can also occur when cycles look regular.¹

Cycle length itself is linked to fertility. In a North American cohort of 2,189 women trying to conceive, cycles shorter than 25 days were associated with lower fecundability (the chance of conceiving per cycle) than cycles of 27 to 29 days, though the estimate was imprecise; women whose cycles never became regular after their first period had slightly lower fecundability too.³

What causes irregular periods?

ASRM lists the most common causes of ovulatory dysfunction as PCOS, obesity, perimenopause, weight gain or loss, strenuous exercise, thyroid dysfunction and high prolactin; the WHO also names hormonal imbalances of the hypothalamus and pituitary.²⁴ Your clinician works through them in order:

• PCOS. A leading cause of infrequent ovulation; see PCOS and getting pregnant.
• Thyroid disorders. TSH identifies thyroid problems, which can impair fertility when untreated.²
• High prolactin. ASRM recommends a prolactin test when periods are infrequent or absent, or there is nipple discharge.²
• Hypothalamic causes. Low or normal FSH with low estradiol suggests the brain’s signal to the ovaries is low, which can follow weight loss or strenuous exercise.²
• Ovarian insufficiency. High FSH with low estradiol suggests the ovaries are running low on follicles.²
• Androgen-related conditions. With signs of androgen excess, ASRM adds testosterone and 17-hydroxyprogesterone to check for a rare adrenal condition.²

Short cycles and bleeding between periods

Irregular does not only mean long. ASRM lists cycles shorter than 25 days and bleeding between periods alongside infrequent and absent periods as reasons to start fertility testing without delay.² In the PRESTO cohort, cycles shorter than 25 days were associated with a per-cycle chance of conceiving about 0.81 times that of 27 to 29 day cycles, though the confidence interval was wide (0.54 to 1.22).³

What to bring to your appointment

• Start dates of your periods for the last six to twelve months
• Any ovulation test or tracker results, including cycles with no surge
• Any symptoms: excess hair growth, acne, nipple discharge, weight change, hot flushes
• Medications and any recent hormonal contraception
• Your partner’s semen analysis, or a plan to get one

These map onto the tests ASRM recommends for irregular cycles: TSH, prolactin, FSH with estradiol, and androgen tests when there are signs of excess.²

Can you get pregnant with irregular periods?

Yes, often. Irregular ovulation reduces how many chances you get, but many of the causes are treatable, and ovulation can be induced with medication when needed. For PCOS-related anovulation, the 2023 guideline makes letrozole the first-line ovulation medication.¹ In a 750-woman trial, letrozole gave a 27.5% live birth rate against 19.1% for clomiphene.⁵ If oral medication does not work, the guideline lists gonadotropin injections or laparoscopic ovarian surgery as second-line options and IVF as third-line.¹ For thyroid or prolactin problems, treating the underlying condition is the first step.² Treatment depends on the cause, so testing comes first.

How to track ovulation with irregular cycles

• LH strips. They detect the hormone surge before ovulation. With long cycles, start earlier and expect to test for more days, so price per strip matters.
• Trackers and monitors. Multi-hormone monitors and temperature-based trackers can show both the surge and signs that ovulation happened.
• Confirming ovulation. ASRM says a serum progesterone above 3 ng/mL, measured about a week before your expected period, is presumptive evidence of recent ovulation.² With unpredictable cycles, your clinician can time it from an LH surge.
• A cycle log. Write down each period’s start date and any positive ovulation test. Several months of data make a first appointment far more useful.

When should you see a doctor?

Now, if you are trying to conceive. ASRM says fertility testing should begin without delay for people with irregular cycles, cycles shorter than 25 days, bleeding between periods, infrequent periods or absent periods.² You don’t need to try for 12 months first.

A typical first visit includes your history, an exam, TSH and, depending on your pattern, prolactin, FSH and estradiol, and AMH or an ultrasound follicle count.² A male partner’s semen analysis should start at the same time.² Read when to see a fertility specialist for the full workup.

Can an at-home test help?

Some at-home hormone kits measure FSH, LH, estradiol, prolactin or TSH, and one can give you numbers to bring to a first appointment; LetsGetChecked says any weekday works for its hormone test if your periods are unpredictable. But a kit cannot diagnose the cause or treat it. See the Investigate hub for next steps.