The Menstrual Cycle: What Happens From One Period to the Next
The menstrual cycle is often taught as a neat 28-day chart with four hormone lines rising and falling. The chart is useful, but it can make the physiology feel more complicated than it is.
The cycle is really one coordinated process happening in several places at once. The ovaries are developing an oocyte and changing their hormone output. The uterus is responding to those hormones by shedding, rebuilding and then transforming the endometrium. Above both of them, the hypothalamus and pituitary are constantly adjusting the signals that drive the ovaries.
The purpose is straightforward: develop an oocyte, create the conditions for ovulation, prepare the uterus for possible implantation, and reset if pregnancy does not occur.
Menstruation is therefore not the cycle itself. It is the visible beginning of a new cycle while events in the ovary are already moving on.
The cycle is happening in the ovary and uterus at the same time
It helps to separate the names used for the ovarian cycle from the names used for the uterine cycle, because diagrams often mix them together.
In the ovary, the two broad phases are the follicular phase before ovulation and the luteal phase after ovulation. In the uterus, the endometrium passes through the menstrual, proliferative and secretory phases.
These are not separate timelines. They overlap.
Day 1 is defined as the first day of menstrual bleeding, but it is also the first day of the follicular phase. While the uterus is shedding the functional layer of its endometrium, follicle-stimulating hormone is already supporting the next group of ovarian follicles.
The ovaries contain follicles at different stages of development. A follicle is not simply an oocyte sitting inside the ovary. The oocyte is surrounded by specialised cells, particularly granulosa cells and theca cells, which become important hormone-producing tissue as the follicle matures.
The uterus has a thick muscular wall, the myometrium, and an inner lining, the endometrium. The endometrium itself has layers. The deeper basal layer remains after menstruation and provides the tissue from which the surface functional layer can regenerate. The functional layer is the part that proliferates, becomes secretory and is shed if pregnancy does not occur.
This distinction is useful because the uterus does not have to rebuild its lining from nothing every month. Menstruation removes the hormone-dependent functional layer while the basal layer remains in place.
From menstruation to ovulation: how a follicle becomes dominant
The events that produce a new period begin at the end of the previous cycle. If pregnancy has not occurred, the corpus luteum in the ovary regresses. Progesterone, estradiol and inhibin levels fall. The endometrium loses the hormonal support that had maintained its secretory state, and the functional layer begins to break down.
At the same time, the fall in ovarian hormones reduces negative feedback on the hypothalamus and anterior pituitary. Pulsatile gonadotrophin-releasing hormone (GnRH) from the hypothalamus drives the pituitary release of follicle-stimulating hormone (FSH) and luteinising hormone (LH). FSH begins to rise enough to support a new cohort of follicles.
This is one of the easiest places to get confused when learning the cycle. Menstruation is not a quiet period in which nothing is happening hormonally. The uterus is shedding while the next ovarian phase is already under way.
As follicles develop, LH and FSH work with two different follicular cell populations. LH acts mainly on theca cells, stimulating the production of androgen precursors. FSH acts mainly on granulosa cells, where it increases aromatase activity. Aromatase converts those androgen precursors into oestrogens, particularly estradiol. This interaction is often called the two-cell, two-gonadotrophin model.
Several follicles may respond initially, but usually one becomes dominant. As the dominant follicle grows, it produces increasing estradiol and inhibin B. These hormones feed back on the pituitary and help bring FSH levels down. The dominant follicle is able to continue developing despite this lower FSH environment, while most of the other recruited follicles undergo atresia.
The rising estradiol is doing two jobs at once. In the uterus it drives the proliferative phase, stimulating the endometrium to rebuild after menstruation. In the reproductive tract it also alters cervical mucus, making it thinner and more favourable to sperm movement as ovulation approaches.
For most of the follicular phase, estradiol contributes to negative feedback. Near ovulation that relationship changes. Sustained high estradiol from a mature dominant follicle produces positive feedback at the hypothalamic-pituitary level and triggers the mid-cycle LH surge.
The LH surge is the event that pushes the mature follicle towards ovulation. It triggers final oocyte maturation, luteinisation of follicular cells and biochemical changes in the follicle wall. Ovulation usually occurs roughly a day and a half after the LH surge begins, although the precise timing varies.
The follicle then ruptures at the ovarian surface and releases the oocyte-cumulus complex into the peritoneal cavity beside the uterine tube. The fimbriae help direct it towards the tubal opening, while ciliary movement and smooth-muscle contractions help move it along the tube. If fertilisation occurs, it most commonly happens in the ampulla of the uterine tube.
This is why “ovulation happens on day 14” is not a rule. A 28-day diagram uses day 14 because it is an easy teaching model. In real cycles, the follicular phase is the more variable part. The NHS describes ovulation as commonly occurring around 10 to 16 days before the next period, so someone with a longer or shorter cycle will not necessarily ovulate on the same calendar day each month.
The fertile period is also wider than the few hours in which the oocyte remains viable. Sperm can survive for several days in favourable cervical mucus, so conception can occur following intercourse before ovulation as well as close to the moment of ovulation itself.
After ovulation, the corpus luteum changes what the uterus is doing
Once the oocyte has been released, the follicle does not simply disappear. The remaining granulosa and theca cells reorganise into the corpus luteum, a temporary endocrine structure.
The corpus luteum produces large amounts of progesterone, as well as estradiol and inhibin A. Progesterone now becomes the dominant hormonal influence on the uterus.
Estradiol has already rebuilt the endometrium. Progesterone changes that proliferative tissue into a secretory endometrium. Endometrial glands become more active and tortuous, the stromal tissue changes and the spiral arteries develop further. The endometrium is no longer simply becoming thicker. It is being converted into tissue capable of supporting implantation if an embryo arrives.
Progesterone also changes cervical mucus so that it becomes thicker and less permeable to sperm. It produces a small rise in basal body temperature after ovulation as well. These effects are why fertility-awareness methods sometimes combine cycle timing, cervical mucus changes and temperature patterns rather than relying on a calendar alone.
The luteal phase is usually less variable than the follicular phase. Its duration depends on the functional life of the corpus luteum. Without a pregnancy signal, the corpus luteum begins to regress and progesterone and estradiol concentrations fall.
That hormone withdrawal is central to menstruation. The functional endometrium becomes unstable. Local inflammatory mediators, prostaglandins, changes in the spiral arteries and tissue-degrading enzymes contribute to breakdown of the functional layer. Blood, endometrial tissue and secretions then leave the uterus through the cervix and vagina.
Menstrual fluid is therefore more than blood alone. It contains blood mixed with endometrial tissue, cervical and vaginal secretions and inflammatory mediators. The basal endometrium remains, ready to regenerate when estradiol rises again in the next follicular phase.
If fertilisation has occurred and an embryo implants, the sequence changes. Trophoblastic tissue begins producing human chorionic gonadotrophin (hCG). hCG acts on the corpus luteum and prevents the normal luteal regression, allowing progesterone production to continue during early pregnancy until placental hormone production becomes sufficient to take over.
That is why pregnancy tests detect hCG. The test is detecting a hormone produced by developing trophoblastic tissue after implantation has begun, not merely detecting that a period has failed to arrive.
A 28-day diagram is useful, but the patient is not a diagram
The cycle is often presented as if there is one correct number of days for every person. There is not. Menstrual cycles vary between individuals and can also vary within the same person over time. The follicular phase accounts for much of that variation because the time required to recruit and mature a dominant follicle is not identical in every cycle.
Not every episode of uterine bleeding represents a normal ovulatory menstrual cycle either. Anovulatory cycles can occur, and hormonal contraception can deliberately alter or suppress the normal hypothalamic-pituitary-ovarian pattern. For example, a scheduled bleed during a hormone-free interval with combined hormonal contraception is a withdrawal bleed rather than menstruation following a normal ovulatory luteal phase.
That distinction becomes useful when taking a history. Asking only “When was your last period?” may not tell you whether the person is ovulating, whether the bleeding was expected or whether they are using a method of contraception that changes the pattern.
A menstrual history should build a picture of what is normal for that individual. Useful questions include cycle length, regularity, duration of bleeding, whether the pattern has changed, pain, intermenstrual or post-coital bleeding, contraception, medicines, possibility of pregnancy and associated symptoms. When bleeding is heavy, practical questions about flooding, clots, needing to change protection very frequently, bleeding through clothes or bedding, and the impact on work, study, sleep and daily life are often more useful than trying to estimate an exact volume of blood.
NICE specifically advises that heavy menstrual bleeding should be understood in terms of its effect on quality of life rather than by concentrating only on measured blood loss. Someone can have regular cycles and still have bleeding severe enough to cause iron deficiency, fatigue and major disruption.
Irregular or absent cycles also need context. Pregnancy should always be considered where relevant, but the hypothalamic-pituitary-ovarian axis can also be altered by polycystic ovary syndrome (PCOS), thyroid disease, significant illness, low energy availability, major changes in weight or exercise, some medicines, hormonal contraception and the transition towards menopause.
The physiology helps make sense of these patterns. If ovulation does not occur, there is no normal corpus luteum and therefore no normal luteal progesterone pattern. If the endometrium is exposed to altered or unopposed hormonal signals, the timing and character of bleeding can change. Once you understand what the normal cycle is trying to do, abnormal bleeding and irregular cycles become much easier to reason through.
For nursing practice, the useful question is not “Which day should this patient be on?” It is “What usually happens for this person, what has changed, and which part of the normal physiology might explain the pattern?”
Summary
The menstrual cycle is coordinated by the hypothalamus, anterior pituitary, ovaries and uterus. Day 1 is the first day of menstrual bleeding, but ovarian follicle development is already beginning at the same time.
FSH supports follicular development, while LH and FSH work with theca and granulosa cells to produce estradiol. Rising estradiol rebuilds the endometrium and, once sustained at a high level, contributes to the LH surge that triggers ovulation. After ovulation, the corpus luteum produces progesterone, converting the proliferative endometrium into secretory tissue ready for possible implantation.
If pregnancy does not occur, the corpus luteum regresses, progesterone and estradiol fall and the functional endometrium is shed. If implantation occurs, hCG maintains the corpus luteum during early pregnancy.
The 28-day cycle is a useful teaching model, not a definition of normal. Understanding the sequence and the feedback between hormones is more useful than memorising a fixed calendar.
References
NHS (n.d.) Periods and fertility in the menstrual cycle. Available at: https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
National Institute for Health and Care Excellence (2021) Heavy menstrual bleeding: assessment and management (NG88). Available at: https://www.nice.org.uk/guidance/ng88
Reed, B.G. and Carr, B.R. (2018) ‘The Normal Menstrual Cycle and the Control of Ovulation’, Endotext. Available at: https://www.ncbi.nlm.nih.gov/books/NBK279054/
Skorupskaite, K. et al. (2024) ‘Physiology of GnRH and Gonadotrophin Secretion’, Endotext. Available at: https://www.ncbi.nlm.nih.gov/books/NBK279070/