How Your Cycle Actually Works: A Plain English Guide

You are thinking about trying for a baby, maybe in a few months, maybe next year, and you have realised nobody ever drew the whole map for you. You know the words ovulation and luteal phase, but they sit there without a picture attached. The aim of this post is to give you that picture, so the rest of the decisions waiting for you, when to track, when to test, when to ask for help, sit on something solid.

Why cycle literacy is the first fertility skill

Most of us were taught period management, not cycle physiology. We were shown how to use a tampon and warned about cramps, and that was the curriculum. The thing inside that bleed, the four-week conversation between your brain and your ovaries, was treated as background noise.

It is not background noise. It is the engine. The cycle is what makes pregnancy possible, and every fertility decision you will make over the next year, what to track, when to test, when to ask for a referral, depends on understanding which week your body is in and what it is trying to do that week.

There is also a practical reason this matters first. Tracking is only useful if you know what you are tracking. An app that pings you on "day 14" without telling you why is just noise. A basal body temperature chart you cannot interpret is just dots. The reason this post is the pillar of the Before You Start section is that every other piece of cycle work, every OPK reading, every progesterone test, every conversation with a doctor, depends on the same underlying map.

One acknowledgment before we begin. A meaningful fraction of readers are coming to this from somewhere very specific: you have just stopped, or are about to stop, hormonal contraception. The pill, the implant, the coil, the injection. Your underlying cycle has been masked for years, not absent, and the question on your mind is some version of "how do I get pregnant with the implant in your arm still there, or just out". I will come back to that question directly later in the post. It deserves a real answer, not a line.

The four phases in plain English

A normal cycle has four working parts. They run in order, every cycle, and each one has a single job.

Menstrual phase runs roughly from day 1 to day 5. This is the bleed itself. Hormones are at their lowest point of the cycle, and follicle stimulating hormone (FSH) from the pituitary starts to rise in the background, getting the next batch of follicles ready. Day 1 of your cycle is the first day of full red flow, not spotting, not a brown smear the day before. We will use that definition all the way through.

Follicular phase runs from day 1 (yes, it overlaps the bleed) to the day of ovulation, and this is where most of the cycle's length variability lives. In a textbook 28-day cycle this phase is about 14 days. In the real world it can be anywhere from 10 to 21 days and still be normal. FSH recruits a cohort of follicles in the ovary. One of them pulls ahead and becomes dominant. That dominant follicle produces oestradiol, which climbs steadily across the phase. Oestradiol is doing three jobs at once: it is thickening the endometrial lining, producing fertile cervical mucus, and slowly setting up the next event.

Ovulation is the next event. It is a single 12 to 24 hour window, not a phase in the usual sense, in which the dominant follicle ruptures and releases the egg. The trigger is a sharp luteinising hormone (LH) surge from the pituitary, prompted by oestradiol crossing a threshold. Ovulation happens roughly 24 to 36 hours after the LH surge begins. This is the moment the whole front half of the cycle has been building toward.

Luteal phase runs from ovulation to the day before your next period. Unlike the follicular phase, this one is fairly fixed, almost always 11 to 14 days. The empty follicle becomes the corpus luteum and starts producing progesterone, which stabilises the endometrium for implantation. If an embryo implants, the corpus luteum gets a chemical "stay alive" signal from the embryo and keeps producing progesterone. If it does not, the corpus luteum dies on schedule, progesterone falls off a cliff, the lining sheds, and day 1 starts again.

That is the whole cycle. Four parts, one job each: shed, recruit, release, hold. Everything else, the hormones, the tests, the symptoms, the timing, is detail on top of that frame.

The hormones doing the work

Six hormones do most of the heavy lifting. You do not need to memorise them, but recognising the names will make every doctor conversation easier.

GnRH comes from the hypothalamus, the small part of the brain that sits above the pituitary. It is the pacemaker, releasing in pulses that tell the pituitary when to release FSH and LH. Stress, sleep loss, severe undereating, and intense training all act on this pacemaker, which is why life events can shift a cycle.

FSH is released by the pituitary and recruits the cohort of follicles at the start of the cycle. A baseline FSH level drawn in the first few days of the cycle is one of the standard tests of ovarian function.

Oestradiol is the main oestrogen the ovary makes. It climbs across the follicular phase, builds the endometrium, produces the slippery egg-white cervical mucus that lets sperm survive, and eventually drives the LH surge.

LH is the surge that triggers ovulation. This matters in a practical way because ovulation predictor kits (OPKs) detect LH in urine. A positive OPK tells you the surge has begun, which means ovulation is coming in the next 24 to 36 hours. I have spent more clinic time correcting the OPK-equals-ovulation assumption than almost anything else in early TTC visits. A positive OPK is not proof you ovulated. It is the warning bell that ovulation is on its way. Many of the couples I see who mistime intercourse for two or three cycles do it because nobody told them this clearly.

Progesterone is made by the corpus luteum in the luteal phase. It does two things worth holding in mind: it stabilises the endometrium so a pregnancy can implant, and it is the hormone responsible for most of the symptoms people misread as early pregnancy signs. Breast tenderness, bloating, mild fatigue, mood shifts: all progesterone, in pregnant and non-pregnant cycles alike.

hCG is only present when a pregnancy is implanting. It rescues the corpus luteum, keeps progesterone climbing, and is the hormone pregnancy tests detect. There is no "low level" of hCG floating around in non-pregnant cycles.

The reason it matters to know which hormone is doing what is that fertility tests are organised around them. AMH and antral follicle count are early-follicular tests of ovarian reserve. Day 3 FSH and oestradiol are baseline ovarian function tests. LH surge is what an OPK measures. Mid-luteal progesterone confirms whether ovulation actually happened. Beta-hCG is the pregnancy test. When you hear a doctor order one of these, you can place it on the map.

The fertile window: what is actually fertile about it

Here is the part of cycle physiology that has the biggest day-to-day consequence, and the part the internet gets most wrong.

The fertile window is not "the day of ovulation". The fertile window is roughly six days long: the five days before ovulation, plus the day of ovulation itself. Peak fertility is the two to three days before ovulation, not the day of.¹

The reason is biology, not folklore. Sperm can survive in fertile cervical mucus for up to five days. The egg, by contrast, is viable for only about 12 to 24 hours after it is released. If you wait until you are certain ovulation has happened, you have probably already missed it. The most fertile pattern is sperm waiting in the reproductive tract when the egg arrives, not sperm chasing the egg afterwards.⁴

This is why I tell couples to think in days, not hours. Intercourse every one to two days through the fertile mucus window has a higher per-cycle pregnancy rate than trying to "hit" ovulation precisely.⁷ Fertile mucus, that slippery, stretchy, egg-white texture, is itself a fertility signal: it appears under rising oestradiol, peaks around ovulation, and is the single most useful real-time marker of where you are in your cycle.

Now: day 14. Almost every chart you have ever seen places ovulation on day 14. This is true if and only if you have a textbook 28-day cycle. In a real-world dataset of over 600,000 cycles analysed by Bull and colleagues, only about 13% of cycles were exactly 28 days, and 46% of cycle-length variation between consecutive cycles was 7 days or more.² Day 14 is a textbook abstraction. The day you ovulate is whatever day your follicular phase happens to end.

What this means practically: if your cycle was 32 days last month and 27 days this month, the assumption that you ovulated on day 14 of either is probably wrong, and timing intercourse based on that assumption will keep missing. The fix is to anchor on signs your body gives you, fertile mucus, OPK surge, post-ovulatory temperature shift, rather than calendar arithmetic.

Why cycles vary, and what is still normal

A normal adult cycle is anywhere between 21 and 35 days long.³,⁶ That is the ACOG range, the NICE range, and the range I use in clinic. Inside that range, between-cycle variation of up to 7 to 9 days is common, even in people who would describe themselves as "regular".²

The variability mostly lives in the follicular phase, the first half. Anything that delays ovulation (stress, illness, travel, big sleep changes, intense training, sudden weight change) makes the cycle longer. The luteal phase, the second half, is much more stable: 11 to 14 days in most cycles, in most people, most of the time.

A handful of conditions reliably shift cycle length outside the 21 to 35 day range. Polycystic ovary syndrome (PCOS) is the most common, causing long or skipped cycles because ovulation is sporadic. Thyroid disease in either direction can do it. Hyperprolactinaemia, often from medication or a small pituitary adenoma, can do it. Perimenopause does it eventually for everyone. Recent contraception use can do it temporarily. These are the categories worth knowing about, not so you can self-diagnose, but so that when a doctor asks the right questions you understand why.

For a fuller red-flag list and the specific thresholds that trigger a workup, the companion post in this section, why cycle length varies, walks through it in detail.

What changes when you come off contraception

This is the part of the post a lot of you came for, so I want to answer it directly before the cycle map gets any more detailed.

Hormonal contraception does not "use up" your eggs. It does not "hide" infertility. It pauses ovulation by overriding the FSH and LH signals from your pituitary. When the contraception comes out, those signals come back, on a schedule that depends on which method you used.

Combined pill (oestrogen and progestogen): Ovulation usually returns within 1 to 3 cycles of stopping, and pregnancy rates over 12 months are essentially identical to people who never used the pill.⁵ Some people ovulate in the first cycle off. Some take three cycles to settle. Both are normal.

Progestogen-only pill (the mini-pill): Similar to the combined pill in most cases. Return of ovulation is usually rapid.

Hormonal IUD (Mirena, Kyleena, Liletta, Jaydess): Once the device is removed, the systemic progestogen clears quickly. Cycles often return within one to two months.

Implant (Nexplanon, Implanon): This is the one most readers are searching for. The implant is a small plastic rod sitting under the skin of the upper arm that slowly releases etonogestrel, a progestogen that suppresses ovulation. You cannot get pregnant with the implant still in your arm. Removal is the first step. The procedure takes a few minutes at a GP or family planning clinic. Once the rod is out, etonogestrel levels fall rapidly, and ovulation typically returns within 1 to 3 cycles, with no measurable effect on long-term fertility compared to people who never used it.⁵ If you are trying to plan timing: it is reasonable to book the removal a month or two before you want to start trying, give the cycle one or two months to declare itself, then start tracking. The first cycle off the implant is not necessarily fertile and is not necessarily a baseline.

Depot-Provera (the injection): This one is the slowest. The median time to return of fertility after the last injection is around 5 to 6 months, with some people taking up to a year.⁵ If you used Depot and your cycles are still absent at 9 to 12 months, that is worth a GP conversation.

The headline across all methods is the same: previous use of hormonal contraception does not damage long-term fertility. The methods differ only in how quickly the underlying cycle resurfaces. Track patterns over two to three cycles before assuming anything about your baseline.

How a doctor reads your cycle

A short translation guide between what you see at home and what we see in clinic.

Day 1 is the first day of full red flow. Not spotting. Not a brown smear. Full flow. If you bleed lightly on Tuesday evening and heavily on Wednesday morning, day 1 is Wednesday.

Cycle length is counted from day 1 of one period to the day before day 1 of the next. So a cycle that runs from 1 May to 28 May is a 28-day cycle (day 1 = 1 May, next day 1 = 29 May).

Mid-luteal progesterone is the standard test we use to confirm ovulation happened. In a textbook 28-day cycle that is day 21. In a 32-day cycle it is day 25. The rule is simpler than the dates suggest: 7 days after suspected ovulation, or 7 days before the next expected period, whichever you can pin down. A value above roughly 30 nmol/L (10 ng/mL) suggests ovulation occurred in that cycle.

AMH (anti-Müllerian hormone) is a blood test that estimates ovarian reserve. It is a useful planning number, but it is not a fertility predictor and it is not a "deadline". A low AMH does not mean you will not conceive; it means we plan in shorter windows. A high AMH is not a guarantee. We will return to AMH in detail in Section 2 of the library, when getting diagnosed is the active question.

What is normal, what is a red flag

Normal looks like this. Cycles between 21 and 35 days. Periods lasting 2 to 7 days. Some fertile mucus visible mid-cycle. Mild mid-cycle pain on one side (mittelschmerz) in some cycles. Some breast tenderness and mood shifts in the luteal phase. Variation of up to 7 to 9 days between cycles is still within the normal envelope.

The standard advice is to try to conceive for 12 months before asking for a workup if you are under 35, or 6 months if you are 35 or older.³ That guideline assumes a typical cycle and no other red flags. If you have any of the following, do not wait the standard window:

• Cycles consistently longer than 35 days or shorter than 21
• No period for 3 or more months without contraception or pregnancy
• Known PCOS, endometriosis, or history of pelvic surgery
• A partner with a known fertility issue (previous testing, varicocele, prior chemotherapy)
• Severe pelvic pain disrupting daily life
• Bleeding between periods or after sex

These shift the calculus. We cover the specifics in signs you need fertility help sooner.

What to do this month

Concrete next steps that fit on a sticky note.

1. Mark day 1 of your next period (full flow) on a calendar or in an app.
2. Plan to track at least two to three full cycles before drawing any conclusions about pattern.
3. If you recently stopped hormonal contraception, give the cycle one to three months to declare itself before reading anything into the numbers.
4. Start a prenatal vitamin with folic acid 400 micrograms now if you are not already taking one. The neural tube closes very early in pregnancy, often before the first missed period, and folate is one of the few supplements with high-quality evidence for benefit. The preconception checklist covers the rest of the pre-trying setup.

You are not behind by doing this carefully. You are not "losing" cycles by observing. The first job of the next two to three months is data, not pressure.

What's next

• If you are ready to start trying once you have a baseline: when to start trying
• If you want the phase-by-phase breakdown of follicular, ovulation, and luteal: cycle phases explained
• If your cycles look irregular or you have known or suspected PCOS: why cycle length varies, normal ranges and red flags
• If you are 35 or older, or have been off contraception for 6+ months without a period: when to see a fertility doctor

Sources

1. Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine 1995;333(23):1517-1521. https://www.nejm.org/doi/full/10.1056/NEJM199512073332301
2. Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digital Medicine 2019;2:83. https://www.nature.com/articles/s41746-019-0152-7
3. National Institute for Health and Care Excellence. Fertility problems: assessment and treatment. NICE Clinical Guideline CG156. Published 2013, updated 2017. https://www.nice.org.uk/guidance/cg156
4. Practice Committee of the American Society for Reproductive Medicine. Optimizing natural fertility: a committee opinion. Fertility and Sterility 2017;107(1):52-58. https://www.asrm.org/practice-guidance/practice-committee-documents/optimizing-natural-fertility-a-committee-opinion-2022/
5. Hassan MAM, Killick SR. Is previous use of hormonal contraception associated with a detrimental effect on subsequent fecundity? Human Reproduction 2004;19(2):344-351. https://academic.oup.com/humrep/article/19/2/344/2356397
6. American College of Obstetricians and Gynecologists. ACOG Committee Opinion No. 651: Menstruation in girls and adolescents, using the menstrual cycle as a vital sign. Obstetrics & Gynecology 2015;126(6):e143-146. Reaffirmed 2020. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2015/12/menstruation-in-girls-and-adolescents-using-the-menstrual-cycle-as-a-vital-sign
7. Stanford JB, White GL, Hatasaka H. Timing intercourse to achieve pregnancy: current evidence. Obstetrics & Gynecology 2002;100(6):1333-1341. https://pubmed.ncbi.nlm.nih.gov/12468181/