Irregular periods: what are the causes?
A cycle is irregular when its length changes by more than 7 days from one month to the next. The most common causes are adolescence, stopping contraception, stress, PMOS, perimenopause and thyroid disorders. Most of them can be spotted simply by logging your period dates for a few months.
What does irregular actually mean?
No woman has perfectly identical cycles. A 27-day cycle followed by a 30-day cycle is normal. We talk about irregular cycles when the gap between the shortest and the longest exceeds 7 days over a few months, or when it becomes impossible to predict your period within a week.
What varies, in almost every case, is the first half of the cycle: the time the body takes to ovulate. The second half, between ovulation and your period, is much more stable, around two weeks. An irregular cycle is therefore almost always an ovulation that comes early, late, or not at all.
Causes that often resolve on their own
- The first years. After your first period, the cycle can take two to three years to find its rhythm.
- Stopping hormonal contraception. The body takes back control of ovulation, and that can take several months.
- Stress, fatigue, a life change. Exam season, a move, a loss will shift ovulation. The rhythm returns when the pressure drops.
- Postpartum and breastfeeding.
- Travel, jet lag, a passing illness.
Causes that deserve a check-up
- PMOS (formerly PCOS). It is the leading cause of irregular cycles in women of reproductive age. It affects roughly one woman in ten, and the large majority of them have irregular, often long, cycles. It sometimes comes with acne, hair growth, difficulty losing weight.
- Perimenopause. In the years before menopause, cycles get shorter, then longer, then further apart.
- The thyroid. Whether it works too much or too little, it disrupts the cycle.
- Excess prolactin. This hormone, the breastfeeding one, blocks ovulation when it is too high outside of pregnancy.
- Significant weight loss or very intense sport. The body pauses reproduction when it lacks energy.
Endometriosis, on the other hand, does not generally make the cycle irregular. It makes periods more painful, heavier, longer, and causes pain outside of periods. If you have those symptoms with regular cycles, that is a different lead.
How to find out what is going on for you
The answer comes down to one habit: log the first day of each period, every month, even when it comes late. After three or four cycles, a pattern appears, and your doctor gets much more out of it than "I think it is irregular".
Also log what comes with the cycle: pain, bleeding outside of periods, acne, fatigue. It is often the combination that points to the diagnosis.
When to see someone
- Your cycles vary by more than 7 days for more than three months, outside of the situations that explain it (adolescence, recently stopped contraception, breastfeeding).
- You go more than three months without a period.
- Your cycles are irregular and you have acne, increasing hair growth, or trouble losing weight: that is the classic PMOS trio.
- You are trying to conceive.
- You bleed between periods or after sex.
Lovae is a wellbeing app, not a medical device. It helps you keep track of your cycles; it does not diagnose.
Sources
- Irregular periods (NHS)
- Stopped or missed periods (NHS)
- Comprendre le syndrome des ovaires polykystiques : définition, causes et fréquence (Assurance Maladie, ameli.fr)
- Endométriose : définition et facteurs favorisants (Assurance Maladie, ameli.fr)
Pages consulted on 4 September 2026.