Your cycle begins on the first day of your period and ends on the day before the next one. During this time, a follicle matures in the ovary, ovulation releases an egg, and the uterine lining prepares for a possible pregnancy. If no pregnancy occurs, everything starts over with the next bleed. This process is controlled by four hormones: FSH, LH, estradiol, and progesterone (Reed and Carr 2018).
Data box: The cycle in numbers
In an analysis of 612,613 cycles from a cycle app, a cycle lasted 29.3 days on average, and only 13% of cycles were exactly 28 days long. The time until ovulation averaged 16.9 days and was between 10 and 30 days in 95% of cycles. The time after ovulation averaged 12.4 days, and bleeding 4 days (Bull 2019, funded by the app provider).
What is the menstrual cycle?
The menstrual cycle covers the days from the first day of one bleed to the start of the next (Reed and Carr 2018). Cycle day 1 is therefore always the first day of your period (WHO 2022).
The rhythm comes from the brain. The hypothalamus releases the control hormone GnRH in pulses, the pituitary gland responds with FSH and LH, and the ovaries then produce estradiol and progesterone. These in turn act back on the brain and the uterus. Medically, the cycle has two halves: the follicular phase, from the first day of your period to ovulation, and the luteal phase, from ovulation to the next period (Reed and Carr 2018). The period is therefore part of the follicular phase. In everyday life, the cycle is usually divided into four stages: menstruation, follicular phase, ovulation, and luteal phase.
The phases of the menstrual cycle at a glance
The days in the table refer to a 28-day cycle. In the app analysis, such cycles had a follicular phase of 15.4 days and a luteal phase of 12.6 days on average (Bull 2019). If your cycle is longer or shorter, it is mainly ovulation that shifts.
| Phase | Typical days in a 28-day cycle | Leading hormones | What happens in the body | What you may notice |
|---|---|---|---|---|
| Menstruation | Day 1 to about 5, 2 to 7 days is normal | Progesterone and estradiol low, FSH rising | The upper layers of the uterine lining detach and are shed | Bleeding, often cramps in the lower abdomen |
| Follicular phase | Day 1 to ovulation, on average a little over 15 days | FSH, then rising estradiol | Several follicles grow, one becomes dominant, the lining builds up again | Cervical mucus becomes visible or can be felt as moisture once the fertile time begins |
| Ovulation | 10 to 16 days before the next period, so about day 12 to 18 in a 28-day cycle | LH peak, an estradiol peak shortly before it | The follicle ruptures, the egg travels into the fallopian tube and can be fertilized for about 24 hours | Clear, slippery, stretchy mucus, in some women a one-sided pulling pain in the lower abdomen |
| Luteal phase | From ovulation to the period, 11 to 17 days is normal | Progesterone, plus estradiol | The corpus luteum produces progesterone and prepares the lining for implantation | Basal body temperature 0.2 to 0.5°C higher, mucus sticky or dry, often breast tenderness or mood swings before the period |
Sources for the table: Reed and Carr 2018, Bull 2019, NHS, WHO 2022, ASRM 2026, familienplanung.de 2026, MedlinePlus 2025.
Menstruation
Without a pregnancy, the corpus luteum winds down, and progesterone and estradiol fall. The small spiral arteries of the lining constrict, the upper layers receive less blood and detach. At the same time, the lining releases prostaglandins, which make the uterus contract (Reed and Carr 2018). You feel these contractions as period pain.
According to the NHS, bleeding lasts 2 to 7 days, usually about 5, and is heaviest on the first two days. In total, about 20 to 90 ml of blood is lost (NHS).
By then, the next round has already begun: within 2 days of the start of bleeding, estrogen from the newly growing follicles stimulates the lining to renew itself (Reed and Carr 2018).
Follicular phase
In the first 4 days of the cycle, FSH recruits a group of follicles to grow. Between days 5 and 7, one of them is selected. From about day 8, it becomes dominant and holds back the others, which regress. This dominant follicle produces more and more estradiol (Reed and Carr 2018). The lining thickens. As soon as you see cervical mucus or feel it as moisture, you may be fertile (WHO 2022).
The follicular phase is the part of the cycle that varies most. In the app analysis, it lasted between 10 and 30 days in 95% of cycles, while the luteal phase lasted between 7 and 17 days (Bull 2019). So if your period comes earlier or later than usual, it is usually ovulation that has shifted.
Ovulation
When estradiol reaches a critical level, its effect on the pituitary gland reverses: instead of holding LH back, it triggers the steep LH surge. By then, the follicle is almost always larger than 15 mm on ultrasound (Reed and Carr 2018). In a WHO study in which the ovaries were examined directly during surgery, estradiol peaked a median of 24 hours before ovulation. The LH surge began a median of 32 hours before (WHO 1980).
After ovulation, the egg can be fertilized for about 24 hours (familienplanung.de 2026). Around ovulation, cervical mucus is clear, slippery, stretchy, and wet (WHO 2022). Some women feel ovulation as a one-sided pain in the lower abdomen, called mittelschmerz, which can switch sides from month to month (MedlinePlus 2025). Which signs there are and how reliable they are is explained in the guide Ovulation symptoms.
Ovulation symptoms: signs, pain, and bleeding
Ovulation symptoms at a glance: cervical mucus, LH surge, mittelschmerz, ovulation bleeding, and discharge. What is normal and when to get it checked.
Luteal phase
The empty follicle turns into the corpus luteum. It mainly produces progesterone, which prepares the lining built up by estrogen for an embryo to implant (Reed and Carr 2018). Progesterone peaks 6 to 8 days after ovulation (ASRM 2026) and raises body temperature slightly (Bull 2019): during this time, basal body temperature is 0.2 to 0.5°C higher, and cervical mucus becomes sticky or dry (WHO 2022). In the weeks before their period, most women have symptoms such as breast tenderness, bloating, tiredness, or mood swings, at least some of the time (NHS).
The luteal phase lasts 14 days on average, and 11 to 17 days is normal (ASRM 2026). The corpus luteum only keeps working as long as LH supports it or an embryo produces the pregnancy hormone hCG. Otherwise it regresses, progesterone falls, and the next period begins (Reed and Carr 2018).
If the second half of the cycle lasts 10 days or less, ASRM speaks of a luteal phase defect (ASRM 2026). What lies behind it and what the evidence on progesterone shows is covered in the guide Luteal phase defect.
Luteal phase defect: low progesterone and fertility
Luteal phase defect: what the corpus luteum does, what progesterone levels show and when progesterone helps according to guidelines.
Hormones in the menstrual cycle
FSH and LH come from the pituitary gland, estradiol and progesterone from the ovary. Together they determine when a follicle matures, when ovulation happens, and when the period starts.
| Hormone | Produced in | Role in the cycle | Course over the cycle |
|---|---|---|---|
| FSH (follicle-stimulating hormone) | Pituitary gland | Makes a group of follicles grow | Rises at the end of the previous cycle, falls in the follicular phase, small peak shortly before ovulation |
| LH (luteinizing hormone) | Pituitary gland | Triggers ovulation, keeps the corpus luteum going | Low at first, rises from the middle of the follicular phase, steep peak before ovulation |
| Estradiol (the main estrogen) | Growing follicle, later the corpus luteum | Builds up the lining, triggers the LH peak | Rises until about one day before ovulation, falls, second smaller rise in the luteal phase |
| Progesterone | Corpus luteum | Prepares the lining for implantation, raises temperature | Low until ovulation, peak 6 to 8 days after, drops before the period |
Sources for the table: Reed and Carr 2018, WHO 1980, ASRM 2026, Bull 2019.
The hormones keep each other in check. Rising estradiol and inhibin B, another hormone from the growing follicle, push FSH down in the follicular phase. Only when estradiol has been high for long enough does the feedback switch, and LH shoots up (Reed and Carr 2018). Because the pituitary gland works in pulses, and the frequency and height of the pulses change with the phase of the cycle, a single hormone level is always a snapshot. Which reference ranges apply in each phase and what high or low levels can mean is covered in the guide LH levels.
For a fertility evaluation, hormones are therefore measured at set times. According to the German-language guideline (Germany, Austria, and Switzerland), the practice measures LH, FSH, prolactin, TSH, male hormones such as testosterone, and AMH on cycle days 2 to 5, or regardless of cycle day if no follicle is maturing (AWMF 015-085). Progesterone is measured in the middle of the second half of the cycle, on day 21 of a 28-day cycle, to confirm ovulation (NICE NG257). A level above 3 ng/ml indicates recent ovulation (ASRM 2026).
AMH is not one of the hormones that control the cycle. It comes from the follicles and plays a role in selecting the dominant follicle (Reed and Carr 2018). In the same woman, it varies by 20% on average over the cycle (Hadlow 2016). AMH is considered a marker of ovarian aging (Harlow 2012), but it does not predict a natural pregnancy (NICE NG257). Typical levels by age are shown in the AMH Chart.
LH levels: chart, normal values, too high or too low
Understand LH levels in blood: reference ranges by cycle phase and for men, what high and low values can mean and what the LH/FSH ratio shows.
How long is a normal menstrual cycle?
Cycles commonly last 21 to 35 days (NHS). The German-language guideline on PMOS (Germany, Austria, and Switzerland) gives an average of about 28 days and considers variations between 25 and 35 days natural (AWMF 089-004). According to familienplanung.de, only about one in ten women has a cycle of exactly 28 days, and the length varies even in the same woman (familienplanung.de 2026). How to determine your cycle length and when a deviation stands out is explained in the Period Calculator, section on normal cycles.
Irregular cycle
According to the guideline, a cycle is considered abnormal if it is repeatedly shorter than 21 or longer than 35 days, or if there are fewer than 8 periods a year. This usually indicates that ovulation is not occurring regularly (AWMF 089-004).
The most common hormonal causes of absent periods are PMOS (formerly PCOS) and a disruption of the control system in the hypothalamus and pituitary gland (AWMF 015-085). This functional hypothalamic amenorrhea is often linked to stress, weight loss, a great deal of exercise, or a combination of these (Endocrine Society 2017). Other causes, and how to estimate your next period anyway, are covered in the Period Calculator, section on irregular cycles. Which treatments help you get pregnant with PMOS is explained in the guide Getting pregnant with PCOS.
Getting pregnant with PCOS: letrozole, metformin, IVF
Getting pregnant with PCOS: letrozole (off-label), metformin (off-label), gonadotropins, IVF. Guidelines, insurance in Germany, Austria & Switzerland.
Fertile days and ovulation in the cycle
You can get pregnant on 6 days per cycle: the 5 days before ovulation and the day of ovulation. In a study of 221 women, pregnancies occurred only after sex during this time (Wilcox 1995). The reason is the different lifespans: sperm can survive for up to 5 days in the cervix and uterus and wait there for ovulation, while the egg can be fertilized for only about 24 hours (familienplanung.de 2026). How high the chance is on each day is shown in the Ovulation Calculator, section on chances.
When these 6 days fall is harder to say. In only about 30% of women did the fertile window fall entirely between cycle days 10 and 17, where rules of thumb place it. Most reached it earlier, some much later, even with a regular cycle. Women with a regular 28-day cycle were most likely to be fertile on days 8 to 15 (Wilcox 2000).
The Ovulation Calculator gives you a first estimate. Signs from your body make it more precise. Cervical mucus shows that the fertile time is beginning (WHO 2022), and the chance of pregnancy is highest when it is slippery and clear (ASRM 2022). An ovulation test detects the LH surge in urine shortly before ovulation. Basal body temperature rises only after ovulation (WHO 2022) and, according to NICE, is not suitable for confirming it (NICE NG257).
Cervical mucus: stages, ovulation, and egg white mucus
Cervical mucus explained: a chart from dry to stretchy, the peak day and ovulation, the stretch test, and what mucus reveals in pregnancy.
Ovulation test: when to test and how to read the result
How to use an ovulation test correctly: from which cycle day, at what time of day, how to recognize a positive result and when sex afterward matters most.
Basal body temperature: how to measure and read your chart
Measure basal body temperature step by step: rules, the 3-over-6 rule for reading your chart, an example table, disruptors and what the method cannot do.
You do not have to hit the exact day. The chance per cycle is highest when you have sex every 1 to 2 days in the fertile window, and having sex even more often does not lower it (ASRM 2022). What hormone monitors, temperature sensors, and apps can do and what they cost is shown in the Fertility tracker comparison.
Fertility tracker comparison: devices, apps and tests
Fertility trackers for trying to conceive compared: measuring principle, first-year cost, evidence, and data privacy of 11 devices, apps, and tests.
Can you get pregnant during, before, or after your period?
Yes, it is possible in all three situations, but the likelihood differs. The bleeding itself does not matter: it neither prevents a pregnancy nor makes one more likely (WHO 2022). The only thing that counts is how far away ovulation is.
| Situation | How likely | Why |
|---|---|---|
| Sex on the first days of your period | very unlikely, below 1% on cycle day 2 (WHO 2022) | Ovulation is usually still more than 5 days away |
| Sex during your period with a short cycle or long bleeding | possible | The fertile window can begin in the first week of the cycle |
| Sex shortly after your period | possible, more likely with each day | Sperm survive up to 5 days and can reach an early ovulation |
| Sex shortly before your expected period | unlikely, but not impossible | Ovulation is usually over, but late ovulation does occur |
During your period
On the first days of bleeding, a pregnancy is very unlikely. According to the WHO, the chance on cycle day 2 is below 1%, and after that it rises day by day, whether you are still bleeding or not (WHO 2022). In the Wilcox study with 213 women who could be evaluated, around 2% were in the fertile window by cycle day 4, and already 17% by day 7 (Wilcox 2000).
Sex during your period makes a pregnancy more likely in two cases:
- A short cycle. Of the women whose cycles usually lasted 27 days or less, about a third had reached the fertile window by the end of the first week of the cycle, compared with only 7% of those with long cycles. The earliest ovulation in the study was on cycle day 8 and led to the birth of a healthy child (Wilcox 2000). With ovulation on day 8, the fertile window begins, by calculation, on day 3.
- Long bleeding. Bleeding lasts 4 days on average (Bull 2019), but it can continue for up to 7 days (NHS). If long bleeding coincides with early ovulation, the last days of bleeding already fall in the fertile window.
Shortly after your period
Getting pregnant shortly after your period is possible, though not very likely. It is most likely to happen with early ovulation or a short cycle (NHS). The reason is sperm: according to familienplanung.de, they survive for up to 5 days, and the NHS even says up to 7 days. Sex on cycle day 7 can therefore lead to a pregnancy if ovulation follows by day 12.
On every day between cycle days 6 and 21, at least 10% of the women in the Wilcox study were in the fertile window (Wilcox 2000). So there are no reliably infertile days right after your period. However, in the older Wilcox study, only 6% of pregnancies could be clearly attributed to sperm that were 3 days old or older (Wilcox 1995). Sex in the days shortly before ovulation therefore counts for more than sex right after bleeding.
Shortly before your period
Shortly before your period, ovulation is usually over if your cycle is regular, and the egg can be fertilized for only about 24 hours (familienplanung.de 2026). Still, a pregnancy is not ruled out. Even women who considered their cycle regular had a 1% to 6% probability of still being in the fertile window on the day of their expected period (Wilcox 2000).
Late ovulation is the reason. In 28-day cycles, there were 7 to 19 days between ovulation and the next period, and exactly 14 days in only 10% of cycles. Of the women whose cycle had not ended after 4 weeks, 4% to 6% were fertile in the fifth week (Wilcox 2000).
Not every bleed is a period. According to the NHS, you should have bleeding between periods or after sex checked by a doctor.
Menstrual cycle and age
In the first year after your first period, irregular cycles are completely normal. One to three years after it, cycles shorter than 21 or longer than 45 days are considered irregular; after that, the limits for adults apply (AWMF 089-004). The cycle is most irregular at both ends of the reproductive years, around the first period and around menopause, because ovulation more often does not take place then (Reed and Carr 2018).
Between 25 and 45, cycles slowly get shorter, by 0.18 days per year of age in the app analysis. This is almost entirely due to the follicular phase; the luteal phase stayed nearly the same in all age groups (Bull 2019). According to the international staging system for reproductive aging (STRAW+10), shorter cycles begin in the late reproductive stage, when fertility starts to decline. Before that, there is a time when the cycle is still as regular as before, but AMH and the number of small follicles are already low (Harlow 2012).
The transition to menopause begins when consecutive cycles repeatedly differ by 7 days or more. If your period stays away for 60 days or longer, the late transition has been reached, in which cycles without ovulation become more frequent (Harlow 2012). In the UK, the last period comes at age 51 on average (NHS).
Fertility declines long before that. According to the German-language guideline (Germany, Austria, and Switzerland), it decreases after age 35 (AWMF 015-085), and at 40 it is about half as high as in the late 20s or early 30s (ASRM 2022). A regular cycle shows you that you are probably ovulating (NICE NG257). It does not show you your ovarian reserve. What applies if you are trying to conceive at 40 or older is covered in the guide Getting pregnant at 40.
Menstrual cycle after stopping the pill
The pill works mainly by preventing ovulation (WHO 2022). As long as you take it, you therefore cannot see what your cycle looks like without hormones. According to the WHO, fertility returns without delay after stopping (WHO 2022).
A cohort study of 17,954 women from Denmark and North America found a short transition period. After the pill and the vaginal ring, it took about 3 cycles until the chance per cycle was as high as in women who had previously used condoms or other barrier methods. After the contraceptive patch it was 4 cycles, after a hormonal or copper IUD and the implant 2, and after the three-month injection 5 to 8 cycles. How many years someone had used hormonal contraception made no difference to the chance (Yland 2020).
So you can get pregnant as early as the first cycle after stopping. If your period does not come and a pregnancy test is negative, talk to your doctor after 3 missed periods in a row (NHS).
Menstrual cycle while breastfeeding
Frequent breastfeeding temporarily suppresses the hormones that trigger ovulation (WHO 2022). As long as your baby is younger than 6 months, you are fully or nearly fully breastfeeding, including at night, and your period has not returned, about 98 out of 100 women do not get pregnant during this time, according to the WHO. When fertility returns after that depends on how much you continue to breastfeed (WHO 2022).
Without breastfeeding, most women do not ovulate for the first time before the 6th week after giving birth, some earlier. In the studies of one review, ovulation preceded the first period in 20% to 71% of cases (Jackson and Glasier 2011). So you can get pregnant before your first period after the birth.
When a cycle disorder should be checked
Have your cycle checked by a gynecologist if
- your cycles are repeatedly shorter than 21 or longer than 35 days, or you have fewer than 8 periods a year (AWMF 089-004),
- your period stays away for 3 months in a row and a pregnancy test is negative (NHS),
- you bleed between periods or after sex (NHS),
- your bleeding is very heavy, meaning you have to change your pad or tampon every 1 to 2 hours, the bleeding lasts longer than 7 days, or you pass blood clots larger than about 2.5 cm (NHS),
- you have severe pain during your period (NHS). Severe period pain can be a sign of endometriosis (WHO 2025); more on this in the guide Endometriosis and fertility.
If you are trying to get pregnant, do not wait the usual 12 months with cycle disorders like these. With irregular cycles, cycles shorter than 25 days, bleeding between periods, or absent periods, the evaluation should begin without delay, according to ASRM (ASRM 2021). Without such signs, the German-language guideline (Germany, Austria, and Switzerland) recommends an evaluation after 12 months without a pregnancy for women under 35, and after 6 months for women over 35. Women over 40 should go to a fertility clinic straight away (AWMF 015-085). According to NICE, a referral to a specialist should be offered at the very first appointment if you are 36 or older, or if a cause is known or suspected in either of you (NICE NG257).
What the evaluation of both partners involves and what health insurance covers is explained in the Infertility guide. The individual tests for women are explained in the guide Fertility tests for women.
Frequently asked questions
What is cycle day 1?
The first day of your period. From there, you count to the day before your next period, and that is your cycle length (Reed and Carr 2018, WHO 2022). Ovulation tests, hormone tests, and cycle calculators always refer to this count.
How many fertile days are there in a cycle?
Six: the 5 days before ovulation and the day of ovulation (Wilcox 1995). Sperm survive up to 5 days, the egg about 24 hours (familienplanung.de 2026). When the 6 days fall varies, even with a regular cycle (Wilcox 2000).
Is a 24-day or 35-day cycle still normal?
Yes. Cycles of 21 to 35 days are common (NHS), and cycles that are repeatedly shorter than 21 or longer than 35 days are considered abnormal (AWMF 089-004). If you are trying to get pregnant, however, ASRM advises an evaluation without a waiting period for cycles shorter than 25 days (ASRM 2021).
Do you ovulate in every cycle?
Not always. If you get your period regularly every month, you are very likely ovulating, according to NICE (NICE NG257). Cycles without ovulation occur mainly in the years after the first period and before menopause (Reed and Carr 2018). Repeatedly very short or very long cycles suggest that ovulation is not occurring regularly (AWMF 089-004).
Can stress make your period stop?
Yes. Stress, major weight loss, and a great deal of exercise, alone or together, can stop ovulation and with it the period. The Endocrine Society then speaks of functional hypothalamic amenorrhea. This diagnosis is made only once other causes have been ruled out (Endocrine Society 2017).
Can you get pregnant without having had a period first?
Yes, because ovulation comes before the bleeding. After giving birth, ovulation preceded the first period in 20% to 71% of cases in studies (Jackson and Glasier 2011). After stopping the pill, too, fertility returns without delay, and even the first ovulation can lead to a pregnancy (WHO 2022).
Sources
Guidelines and position statements
- Diagnostics and therapy before medically assisted reproduction treatment, S2k guideline AWMF 015-085, version 2.0, DGGG, OEGGG, and SGGG, 2026
- Patient guideline: diagnosis and treatment of polyendocrine metabolic ovarian syndrome (PMOS, formerly PCOS), AWMF register no. 089-004, as of June 2026
- NICE NG257: Fertility problems, assessment and treatment, chapter Investigation of fertility problems and management strategies, recommendations 1.18.3, 1.18.6, 1.18.7, and 1.18.9, NICE, 2026
- NICE NG257: chapter Defining infertility and initial assessment, recommendation 1.16.8, NICE, 2026
- Practice Committee of ASRM: Fertility evaluation of infertile women, a committee opinion, Fertil Steril 2021;116:1255-65
- Practice Committee of ASRM and SREI: Optimizing natural fertility, a committee opinion, Fertil Steril 2022;117:53-63
- Practice Committee of ASRM: Diagnosis and treatment of luteal phase deficiency, a committee opinion, Fertil Steril 2026;126:633-9
- Gordon CM et al.: Functional Hypothalamic Amenorrhea, an Endocrine Society Clinical Practice Guideline, J Clin Endocrinol Metab 2017;102:1413-1439
- Family Planning: A Global Handbook for Providers, chapters Combined Oral Contraceptives, Fertility Awareness Methods, and Lactational Amenorrhea Method, WHO and Johns Hopkins Bloomberg School of Public Health, 2022
- Harlow SD et al.: Executive summary of the Stages of Reproductive Aging Workshop + 10 (STRAW+10), Fertil Steril 2012;97:843-851
Literature and studies
- Reed BG, Carr BR: The Normal Menstrual Cycle and the Control of Ovulation, Endotext, updated 5 August 2018
- WHO Task Force on Methods for the Determination of the Fertile Period: Temporal relationships between ovulation and defined changes in the concentration of plasma estradiol-17 beta, luteinizing hormone, follicle-stimulating hormone, and progesterone, Am J Obstet Gynecol 1980;138:383-390
- Wilcox AJ, Weinberg CR, Baird DD: Timing of sexual intercourse in relation to ovulation, N Engl J Med 1995;333:1517-1521
- Wilcox AJ, Dunson D, Baird DD: The timing of the fertile window in the menstrual cycle, day specific estimates from a prospective study, BMJ 2000;321:1259-1262
- Bull JR et al.: Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, npj Digit Med 2019;2:83, funded by the app provider Natural Cycles
- Hadlow N et al.: Quantifying the intraindividual variation of antimullerian hormone in the ovarian cycle, Fertil Steril 2016;106:1230-1237
- Yland JJ et al.: Pregravid contraceptive use and fecundability, prospective cohort study, BMJ 2020;371:m3966
- Jackson E, Glasier A: Return of ovulation and menses in postpartum nonlactating women, a systematic review, Obstet Gynecol 2011;117:657-662
Consumer health information
- Getting pregnant: ovulation and the fertile days, familienplanung.de (Federal Institute for Public Health), as of 7 January 2026
- Periods, NHS, reviewed 5 January 2023
- Fertility in the menstrual cycle, NHS, reviewed 5 January 2023
- Heavy periods, NHS, reviewed 19 September 2024
- PMS (pre-menstrual syndrome), NHS, reviewed 18 June 2024
- Mittelschmerz, MedlinePlus, U.S. National Library of Medicine, review 22 May 2025
- Fact sheet Endometriosis, WHO, 15 October 2025