The basics · 4 min read
Your menstrual cycle is one of the clearest signals of how your reproductive system is working. Knowing the four phases — and your fertile window — helps you read your body with more confidence.
A typical cycle lasts 21 to 35 days, measured from the first day of one period to the first day of the next. Anything regularly outside that range — or persistent irregularity — is worth a conversation.
Menstrual phase (days 1–5). The lining of the uterus is shed if no pregnancy has occurred. Hormone levels are at their lowest.
Follicular phase (days 1–13). The pituitary releases FSH, which encourages a group of follicles in the ovary to grow. One usually becomes dominant. Oestrogen rises as the follicle matures.
Ovulation (around day 14). A surge in LH triggers the mature follicle to release its egg. The egg survives for roughly 12–24 hours. This is the moment of the cycle that defines the fertile window.
Luteal phase (days 15–28). The empty follicle becomes the corpus luteum and produces progesterone, which thickens the uterine lining ready for a possible pregnancy. If there's no implantation, hormone levels fall and the next period begins.
Because sperm can survive in the female reproductive tract for up to 5 days, the most fertile time is the six days ending on ovulation. NICE recommends regular sex every 2–3 days throughout the cycle rather than rigid timing, which tends to be more effective and less stressful.
Periods that are very heavy, very painful, consistently absent or wildly irregular can point to conditions such as PCOS, endometriosis or thyroid imbalance — all of which can affect fertility and are very treatable when identified. If anything in your cycle feels off, it's worth getting checked.
Cycle awareness is a brilliant first step. If you'd like to understand your individual picture in more depth, our fertility testing guide explains what initial investigations look like, or you can book a consultation with our team.
