Your Cycle, Explained: What’s Normal & Why It Varies
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Aug 26, 2025
The menstrual cycle is a natural, hormone-driven process the body uses to prepare for a potential pregnancy, occurring roughly once a month. Although the textbook cycle is said to be 28–29 days, there is wide natural variation and anything from 26 to 35 days is considered normal and healthy.
Most girls begin menstruating between ages 10 and 16, with the average around 12 to 13 years old. Menstruation continues until menopause, which generally occurs between ages 45 and 55 and the average being around 51. Leading up to menopause is a transitional phase called perimenopause, during which cycles often become increasingly irregular.
Cycle consists of three main phases
Follicular phase (typically days 1–14)
This starts on the first day of bleeding (when the doctor asks when did your last cycle start, they mean when was the most recent first day of bleeding). During this time, the pituitary gland releases follicle-stimulating hormone (FSH), prompting the ovaries to develop follicles (tiny sacs that each hold an immature egg). One follicle becomes dominant and begins producing estrogen, which prepares the uterus by thickening its lining.
Ovulation (around day 14)
Ovulation marks the release of an egg from the ovary. This is triggered by a surge in luteinizing hormone (LH), and it represents the most fertile point in the cycle, when pregnancy is most likely to occur if sperm is present.
Luteal phase (around days 15–28)
After ovulation, the cycle enters the luteal phase. The empty follicle transforms into the corpus luteum, which secretes progesterone along with some estrogen. These hormones maintain the thickened uterine lining in case of fertilisation. If the egg is not fertilised, hormone levels fall, leading to the breakdown and shedding of the lining, which is the menstrual period.
Cycle is regulated by hormones
This entire system is regulated by a complex hormonal feedback loop involving the brain and ovaries. The hypothalamus in the brain produces gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to release FSH and LH. These, in turn, act on the ovaries, which produce estrogen and progesterone. Each component of this system constantly adjusts in response to hormonal signals from the others, maintaining overall cycle balance.
Cycles vary a lot
Cycle length and regularity can vary significantly from person to person, and this variation is influenced by a range of biological and lifestyle factors.
Age is a major factor: adolescents often have irregular cycles as their hormonal systems mature, while women approaching menopause typically experience increasing cycle variability as hormone levels begin to shift.
Body weight also plays a key role, since both underweight and overweight individuals are more likely to experience hormonal imbalances that can disrupt normal menstrual patterns.
In addition, lifestyle factors such as high levels of stress, intense physical training, inconsistent sleep and poor nutrition can all interfere with the hormonal regulation of the menstrual cycle, leading to irregular or unpredictable periods.
PMS (Premenstrual Syndrome)
In the days leading up to and during menstruation, it’s completely normal to experience a range of uncomfortable symptoms. This group of symptoms is known as premenstrual syndrome (PMS). While it varies from person to person, PMS can affect your daily life — including concentration, energy levels, emotional balance, and physical performance — and that’s entirely normal.
What are common PMS symptoms?
Abdominal cramps
Breast tenderness or swelling
Fatigue or low energy
Mood swings, irritability or increased emotional sensitivity
Headaches or migraines
Bloating or water retention
Food cravings (especially for sweets or carbohydrates)
Trouble sleeping (insomnia or disrupted sleep)
Anxiety or feelings of sadness
Lower back pain
Difficulty concentrating (“brain fog”)
Digestive changes (constipation or diarrhea)
Breakouts or skin changes
These symptoms usually begin after ovulation (mid-cycle) and improve once menstruation starts. Some people may experience just a few mild symptoms, while others might deal with more intense discomfort that interferes with daily activities.
