The Menstrual Cycle and Ovulation

A menstrual cycle is not simply the days of bleeding. It begins on the first day of a period and continues until the day before the next period starts. During that time, the ovaries and uterus follow linked but distinct patterns.

Day one is the first day of bleeding The first day of menstrual bleeding is counted as day one of the menstrual cycle. Cycle length is measured from day one of one period to the day before the next period begins. A 28 day cycle is often used in diagrams because it is easy to illustrate. It is not a universal standard. Cycle length can vary between people and between cycles. Puberty, approaching menopause, pregnancy, breastfeeding, health conditions, medicines and hormonal contraception can all affect bleeding patterns. A calendar date alone cannot confirm whether ovulation occurred. The ovarian cycle and uterine cycle are linked The ovarian cycle describes changes within the ovaries. The uterine cycle describes changes in the endometrium, which is the lining of the uterus. These cycles occur together because ovarian hormones act on the uterus. The ovarian cycle has a follicular phase, ovulation and a luteal phase. The uterine cycle includes menstruation, rebuilding of the endometrium and preparation for possible implantation. The follicular phase begins with menstruation At the start of a cycle, levels of oestrogen and progesterone are relatively low. Reduced feedback allows the pituitary gland to release more FSH. FSH supports the development of a group of ovarian follicles. Each follicle contains an immature oocyte. Usually one follicle becomes dominant, although the pattern varies. The developing follicle produces increasing amounts of oestrogen. Oestrogen helps rebuild the endometrium after menstruation. It also influences cervical mucus and provides feedback to the brain and pituitary gland. The follicular phase varies more in length than the later luteal phase. This is an important reason ovulation does not occur on the same cycle day for everyone. Ovulation follows a surge in LH As the dominant follicle produces high levels of oestrogen, the feedback pattern briefly changes. This stimulates a rapid rise in LH, called the LH surge. The LH surge triggers final changes within the follicle and the release of an oocyte. This release is called ovulation. The oocyte first enters the pelvic cavity close to the ovary. Fimbriae and movement within the uterine tube help guide it into the tube. Ovulation often occurs around 10 to 16 days before the next period. It does not reliably occur on day 14 unless the cycle and its phases follow that particular pattern. Even people with regular cycles can experience variation. The luteal phase follows ovulation After ovulation, the emptied follicle changes into a temporary structure called the corpus luteum. The corpus luteum produces progesterone and some oestrogen. Progesterone changes the endometrium from a growing lining into a more supportive, secretory tissue. It also affects cervical mucus and causes a small rise in resting body temperature after ovulation. If an embryo implants, early pregnancy signals help maintain the corpus luteum temporarily. If implantation does not occur, the corpus luteum breaks down. Progesterone and oestrogen levels fall. This hormone fall contributes to breakdown and shedding of the functional endometrial lining. Menstruation then marks day one of the next cycle. A period does not prove that ovulation occurred Some bleeding follows a cycle in which ovulation occurred. Other bleeding can occur without ovulation. This is particularly common when menstrual cycles first begin and when approaching menopause. It can also occur with some health conditions or hormone treatments. A regular looking bleed therefore does not prove that an oocyte was released. Likewise, an absent period does not identify one cause. Pregnancy is one possibility, but hormone changes, illness, stress, weight change, medicines and life stage can also matter. Body signs provide estimates, not certainty Cervical mucus often becomes clearer, thinner and more stretchy near ovulation. A small temperature rise usually occurs after ovulation because of progesterone. Urine tests can detect an LH surge. However, an LH surge does not guarantee that an oocyte was successfully released. Calendar calculations estimate likely timing but cannot identify the exact day reliably. These methods may help someone understand their cycle. They should not be treated as certain proof of fertility or as guaranteed contraception without appropriate guidance. Hormone tests depend on timing Hormone levels change throughout the cycle. A blood result can therefore mean different things depending on when it was taken. A clinician may ask about the first day of the last period, usual cycle length and recent bleeding before interpreting results. Some tests are timed relative to ovulation or the expected next period rather than using the same calendar day for everyone. An isolated hormone result cannot assess every part of ovulation, fertility or reproductive health. Menstruation is not a monthly health requirement A spontaneous menstrual cycle reflects a particular pattern of ovarian and uterine hormones. People using some hormonal contraceptives may have lighter bleeding, irregular bleeding or no bleeding. The bleeding during a hormone free interval may be a withdrawal bleed rather than spontaneous menstruation. Not having a monthly bleed is not automatically harmful. Its meaning depends on pregnancy possibility, medicines, life stage, symptoms and the person's circumstances. Important safety boundaries Very heavy bleeding with dizziness, fainting, severe weakness or breathlessness needs urgent assessment. Possible pregnancy with heavy bleeding, severe abdominal pain, shoulder pain, dizziness or collapse requires immediate medical help. Cycle irregularity without these features is usually assessed according to duration, age, pregnancy possibility and associated symptoms.

The menstrual cycle is a coordinated but variable interaction between the brain, pituitary gland, ovaries and uterus. Day 14 is an example, not a universal rule.