Missed period is a red warning light on the dashboard: a signal that your internal
balance has been disrupted.
At our Polish clinic in London, we understand that for a woman, a week of uncertainty can feel like an eternity. We do not follow the “please wait and see if it comes back on its own” approach. If a pregnancy test is negative and the menstrual cycle stops, we look for the cause more deeply. We perform a gynaecological ultrasound and hormone tests to check whether the reason for the missed period is a thyroid disorder, polycystic ovary syndrome (PCOS), or simply excess cortisol.
In this article, we focus on the most common causes of menstrual cycle disruption and explain when you should see a specialist. When is a late period normal and still within the physiological range?
The belief that a normal menstrual cycle must last exactly 28 days is one of the most common myths repeated by patients. In reality, the normal physiological range is much broader and includes cycles lasting from 21 to even 35 days, and this length may vary slightly from month to month. A woman’s body does not function like a precise clockwork mechanism, which is why a shift in bleeding by two, three, or even four days is completely natural and safe. Such minor fluctuations usually result from temporary lifestyle changes, mild infections, or travel, all of which can briefly affect the sensitive hormonal system.
From a medical point of view, we only speak of a true delay requiring diagnostics whenmenstruation fails to appear at least 7 days after the expected date.
Cause No. 1: Pregnancy
Even when using contraception, pregnancy is still possible. A bad case of food poisoning, antibiotics, or a missed pill is enough to reduce contraceptive effectiveness. That is why a missed period should always be the first signal to take a pregnancy test.
However, remember that pharmacy tests are not always reliable. If you take the test too early, the result may be negative despite fertilisation. If your period is late and the test still shows only one line, do not treat this as a certainty. The only 100% reliable answer comes from a blood test measuring beta-hCG levels. It detects pregnancy very early and – unlike home tests – leaves no room for doubt.
Cause No. 2: Weight fluctuations and bad diet
Many women are unaware that body fat is not only an energy warehouse but also an active organ that produces hormones. Fat tissue plays a role in oestrogen metabolism, which is why any extreme change in body weight directly affects your cycle. If you have lost weight rapidly in a short time or have very low body weight (BMI below 18.5), your brain interprets this as an alarm. For the hypothalamus – the brain’s command centre – a lack of fat reserves means “famine.” Your body thinks that pregnancy is a huge
energy expenditure and you may not survive with such limited resources. To protect your life and vital organ functions, it simply switches off the reproductive system. Periods usually return when weight reaches a safe, physiological level. This mechanism also works in the opposite direction. With significant overweight or obesity, excess fat tissue produces too much oestrogen. This creates hormonal confusion in the body. Ovaries receive incorrect signals, which disrupts follicle maturation and often leads to a complete halt of ovulation.
Cause No. 3: Stress and “pregnenolone steal” – how nerves block the ovaries
This is one of the most common causes of missed periods in women who live under chronic tension or are overworked. This mechanism has strong evolutionary justification: in a threat situation, the body saves energy for survival and switches off reproductive functions as an unnecessary energy expenditure.
The key to understanding this process lies in biochemistry. Both the stress hormone (cortisol) and female sex hormones (progesterone) are produced from the same substance called pregnenolone. Under normal conditions, body uses it where it is needed. This changes dramatically under severe stress. In response to stress, the adrenal glands are given absolute priority and use most of the available pregnenolone to produce cortisol. As a result, progesterone production decreases.
This phenomenon is known as “pregnenolone steal.” Without adequate levels of sex hormones, the cycle is suppressed, ovulation does not occur, and menstruation disappears until hormone levels return to normal.
Cause No. 4: Thyroid disorders, prolactin, and PCOS
Your hormonal system is a network of interconnected vessels. If one element stops working properly, the rest of the system collapses almost immediately. Most often, when pregnancy is excluded, three specific problems are responsible for a missed period. Hypothyroidism and Hashimoto’s disease Hypothyroidism (often caused by Hashimoto’s disease) slows down the entire metabolism.
Body goes into energy-saving mode. Because maintaining a pregnancy requires significant resources, with high TSH levels the ovaries often “fall asleep” and cycles become anovulatory. If, in addition to missed periods, you feel constantly cold, sleepy, and have dry skin – check your thyroid.
Prolactin – the stress hormone
This hormone is responsible for milk production after childbirth. During breastfeeding, high
prolactin naturally blocks ovulation (which is why nursing mothers often do not have periods). The problem arises when prolactin levels rise in a non-pregnant woman – most often due to severe, chronic stress or a pituitary adenoma. Body is “tricked” into thinking you are breastfeeding and suppresses fertility. This condition is called hyperprolactinaemia.
PCOS – Polycystic Ovary Syndrome
This is the most common hormonal cause for which patients come to our clinic. In PCOS, the ovaries produce too many male hormones (androgens). Ovarian follicles, in which the egg matures, grow but do not rupture. Instead of ovulation, the follicle turns into a small cyst. If the egg is not released, menstruation does not occur. If your cycles have always been irregular and you have additionally noticed acne or excessive hair growth, PCOS diagnostics are essential.
Cause No. 5: Discontinuation of contraception (post-pill syndrome)
Many women assume that if they had regular cycles before taking the pill, their period will return perfectly on time after stopping it. Reality often looks different. The first bleed after finishing a blister is usually withdrawal bleed (caused by the sudden lack of pill hormones), but the next natural period may take much longer to appear.
Why does this happen? For months or years of hormonal contraception, your ovaries were “asleep.” The pill supplied ready-made hormones, so the brain (pituitary gland) stopped sending signals which stimulate ovulation. When you suddenly cut off the external hormone supply, your reproductive system needs to reboot.
The brain and ovaries must “learn” to communicate with each other again. This process of restoring the natural hormonal balance can take from 3 to even 6 months. During this time, cycles may be irregular, long, or anovulatory. In medicine, this is called “post-pill amenorrhoea.” It is usually a temporary condition and does not mean permanent infertility, but it does require patience and observation.
Cause No. 6: Menopause before 40 and premature ovarian insufficiency
Most women assume that menopause is something they will only worry about after fifty. Meanwhile, medicine recognises a condition called premature ovarian insufficiency (POI), which can affect women as young as 30, and sometimes even younger. This is a situation in which the pool of ovarian follicles is depleted much earlier than biology would predict. If your period disappears and you additionally start to experience typical menopausal symptoms – hot flushes, night sweats, mood swings, or vaginal dryness – do not ignore it.
This may not be “just fatigue.” Your body is sending a signal that oestrogen levels are dropping sharply. Although it may sound like a diagnosis with serious consequences, early detection is crucial. Measuring AMH (ovarian reserve) levels allows us to assess how much reproductive time you have left and whether hormone replacement therapy is needed to protect your bones and heart from the effects of hormone deficiency.
Cause No. 7: Medications and hidden chronic illnesses
Sometimes the culprit is not the reproductive system but your medicine cabinet or another, seemingly unrelated condition. The body treats fertility as a luxury – if it is fighting another disease or burdened by strong medication, the menstrual cycle is often the first to be switched off.
Pay attention to what you are taking. Antidepressants, tranquillizers (psychotropic drugs), and even some allergy or blood pressure medications can increase prolactin levels, which effectively blocks ovulation. Equally insidious are uncontrolled systemic diseases. Untreated diabetes, anaemia, or coeliac disease (gluten intolerance) weaken the body so severely that it suppresses reproductive functions. It often happens that after stabilising blood sugar levels or switching to a gluten-free diet, menstruation returns spontaneously, without any gynaecological intervention.
Missed period – summary
A missed period is rarely a reason for panic, but it is always a signal that should not be ignored. Instead of waiting for months in uncertainty, it is better to find out what the cause is. Often a minor adjustment of thyroid treatment or a change in diet is enough to restore hormonal balance and improve your well-being. But to do that, the cause must first be diagnosed.
At our Polish clinic in London, we know that the lack of a diagnosis is often more stressful than the illness itself. That is why we will not send you home with the advice to “just observe.” We offer solutions: ultrasound, blood tests, and a consultation with a doctor who combines gynaecological and endocrinological expertise. We will help you restore your body’s natural rhythm.
