The moment a period is overdue the usual reaction is to count the days and, soon after, to buy a pregnancy test. But the menstrual cycle is a sensitive system, and plenty of everyday things can nudge it off schedule. Some come from how you live, some from where you are in life, and a few from a medical condition or a prescription. 

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Health conditions behind a late period

Polycystic ovary syndrome (PCOS)
Ovaries normally grow about five follicles each cycle, and these compete until one wins and releases an egg. With PCOS, there are extra follicles, and the process drags on. Ovulation comes late, and since a period follows ovulation, it comes late too. If no egg is released at all, no period follows.

Thyroid disease and diabetes
Thyroid hormones help steer the menstrual cycle, so when they fall out of balance, periods can turn irregular.

Diabetes plays a part as well. Some studies link type 1 diabetes with early menopause. Researchers think the condition ages blood vessels before their time, and the ovaries age along with them. Periods can then become late and unpredictable as menopause approaches.

A pituitary tumour
This is rare, but worth knowing about. A prolactinoma is a growth on the pituitary gland that produces too much prolactin, the hormone that tells the body to make breast milk. Irregular periods together with headaches, blurred vision and milk-like discharge from the breasts, when you are not breastfeeding, are a reason to see a doctor.

Life pressures that shift the cycle

Stress
Distress affects the part of the brain that directs the pituitary gland. That gland in turn controls the hormones that prompt the ovaries to act. When the signal weakens, a period can arrive late.

Disrupted sleep
Your body clock and your hormones are closely tied. Moving from day shifts to nights, or living with long-term insomnia, can knock the hormones out of step and disturb the cycle. A short spell of poor sleep usually sorts itself out once you settle into a steady routine. If your days and nights keep getting mixed up, the trouble can linger.

Big changes in weight
The hypothalamus makes two hormones, FSH and LH, which keep the ovaries working. Rapid weight loss can switch that production off, and without those hormones the cycle stalls. Crash diets, eating disorders and some illnesses can all bring this on. So can weight-loss drugs such as Ozempic (semaglutide) and Mounjaro (tirzepatide).

Putting on weight can interfere too. People with PCOS may be especially sensitive to swings either way, and obesity itself can alter hormone levels enough to affect periods.

Hard training
Elite athletes and keen marathon or triathlon runners are the usual examples, but anyone who suddenly throws themselves into a punishing routine can be affected. Heavy training can leave the body in a constant calorie shortfall while raising the hormones that interfere with the cycle.

Your body, your age

Timing is rarely tidy at either end of reproductive life. In the first few years after periods begin, cycles are still finding their rhythm. Gaps between bleeds can swing widely, which makes it hard for teenagers to predict the next one.

Later on, the pattern can wobble again. Perimenopause, the stretch before menopause, usually begins in a person’s 40s. Along the way, the usual 28-day cycle can stretch so that periods turn up every 36 to 48 days. Some women lose their periods much earlier. If yours stops completely before 40, doctors say it may point to early menopause or primary ovarian insufficiency, where the ovaries stop working as they should.
Medications

Hormonal contraception
The coil (IUD) and the Depo-Provera injection can both make periods irregular or seem late. The pill works differently. Many people assume it regulates the cycle, but it actually overrides your natural hormones. The bleed you get in the week of inactive pills is withdrawal bleeding, not a true period. If you carry on with active pills past the usual 21 days, the womb lining stays stable and does not shed, so there is no bleed. When you move to the inactive pills, hormone levels fall and bleeding starts. Skip those pills and you skip the bleed.

Other prescriptions
Several other drugs can delay a period as well. They include antipsychotics, antidepressants, steroids, hormone therapy and chemotherapy.

(Disclaimer: This article is for general information only and is not a substitute for professional medical advice, diagnosis or treatment. Do not start, stop or change any medication, contraception or diet and exercise routine based on this article without first speaking to a doctor)