The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study
Open accessAbstract
OBJECTIVES: To provide specific estimates of the likely occurrence of the six fertile days (the "fertile window") during the menstrual cycle. DESIGN: Prospective cohort study. PARTICIPANTS: 221 healthy women who were planning a pregnancy. MAIN OUTCOME MEASURES: The timing of ovulation in 696 menstrual cycles, estimated using urinary metabolites of oestrogen and progesterone. RESULTS: The fertile window occurred during a broad range of days in the menstrual cycle. On every day between days 6 and 21, women had at minimum a 10% probability of being in their fertile window. Women cannot predict a sporadic late ovulation; 4-6% of women whose cycles had not yet resumed were potentially fertile in the fifth week of their cycle. CONCLUSIONS: In only about 30% of women is the fertile window entirely within the days of the menstrual cycle identified by clinical guidelines-that is, between days 10 and 17. Most women reach their fertile window earlier and others much later. Women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular.
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The length of the luteal phase may deviate significantly from 14 days.
“Nevertheless, the length of the luteal phase may also deviate significantly from 14 days.”
From Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (2019) · cites this paper as [1]
The fertile window is defined as the day of ovulation and the 5 days preceding it.
“The fertile window, during which there is a probability of conception from unprotected sex, is defined as the day of ovulation and the 5 days preceding it (the time window for sperm survival).1”
From Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (2019) · cites this paper as [1]
The luteal phase length was between 7 and 19 days in a sample of 28 day cycles.
“For example, the luteal phase length was between 7 and 19 days in a sample of 28 day cycles.1”
From Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (2019) · cites this paper as [1]
There is much greater variation in menstrual cycle characteristics than clinical guidelines state.
“but there is much greater variation than this.1,4–7”
From Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (2019) · cites this paper as [1]
Available data suggests that there may be significant variability in fertile days.
“Available data, however, suggests that there may be significant variability in fertile days.1,4”
From Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (2019) · cites this paper as [1]
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Ramírez Mazo, C., Gómez-Echavarría, L., Lema-Pérez, L., & Röblitz, S. (2026) · Journal of Theoretical Biology · doi:10.1016/j.jtbi.2026.112518
- Authors
- Wilcox, A. J.
- Affiliations
- National Institute of Environmental Health Sciences (US)
- Corresponding author
- Wilcox, A. J
- ORCID
- Wilcox, A. J
- Venue
- BMJ
- Publisher
- BMJ
- Published
- 2000-11-18
- Volume
- 321
- Issue
- 7271
- Pages
- 1259-1262
- ISSN
- 0959-8138 (ISSN-L)
- PMC
- PMC27529
- OpenAlex
- W2082464424
- Cited by
- 452
- References
- 28
- Citations by year
- 2026: 24 · 2025: 16 · 2024: 11 · 2023: 23 · 2022: 20 · 2021: 18 · 2020: 21 · 2019: 25 · 2018: 19 · 2017: 19 · 2016: 17 · 2015: 25 · 2014: 18 · 2013: 21 · 2012: 17
- FWCI
- 14.85
- Type
- journal article
- Language
- en
- Open access
- bronze
- doi.org
- Locations
- BMJ (publishedVersion)
- Europe PMC (PubMed Central) (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- Topics
- Health Sciences › Medicine › Public Health, Environmental and Occupational Health › Reproductive Health and Contraception
- Health Sciences › Medicine › Reproductive Medicine › Reproductive Health and Technologies
- Health Sciences › Medicine › Public Health, Environmental and Occupational Health › Reproductive Biology and Fertility
- Keywords
- fertile window, menstrual cycle, ovulation timing, fertility awareness, fertility prediction
- MeSH terms
- Estrogens, Female, Fertility, Humans, Menstrual Cycle, Periodicity, Pregnancy, Progesterone, Prospective Studies
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