By Lynnea Nicholls, DO
Director’s Note: Dr. Lynnea Nicholls wrote this review as part of the FACTS elective during her fourth year of medical school at Rocky Vista University College of Osteopathic Medicine. In her analysis of the study, “Association between menstrual cycle length and ovarian reserve: A systematic review and meta-analysis” by Younis et al., she highlights evidence that women with shorter menstrual cycles (21 to 27 days), even within the normal range, have lower ovarian reserve and reduced fertility outcomes compared with women with 28 to 31 day cycles. This research reinforces the value of fertility awareness-based methods (FABMs) as longitudinal cycle charting can provide meaningful insight into reproductive health and help clinicians recognize trends in cycle length that may prompt earlier fertility counseling and individualized patient care.
Introduction
Many women assume that having regular ovulatory-menstrual cycles throughout their reproductive years indicates preserved fertility. While regularity suggests normal ovulation, evidence shows that it does not necessarily reflect ovarian reserve. As women age, overall cycle length naturally shortens, typically due to a shortened follicular phase. Studies show that the average cycle length decreases from approximately 30 days at age 20 to 27 days by age 40.[1] Despite this phenomenon, cycle shortening has not been widely recognized as an early marker of diminished ovarian reserve.
“As women age, overall cycle length naturally shortens …from approximately 30 days at age 20 to 27 days by age 40.”
Ovarian aging varies significantly among women of the same chronological age, with the timing of menopause differing by up to 20 years.[2] For any given age, two women can differ up to 100-fold in ovarian reserve.[3] Although anti-Müllerian hormone (AMH) and antral follicle count (AFC) are considered reliable biomarkers for assessing ovarian reserve, their ability to predict pregnancy potential remains limited.[4] [5]
A systematic review and meta-analysis by Younis et al. included 11 studies and a total of 12,031 women. It examined whether cycle length within the normal range (21 to 35 days) correlated with ovarian reserve, fecundability, and in vitro fertilization (IVF) outcomes. The authors hypothesized that shorter cycle length, even within normal limits, may signal a reduction in ovarian reserve.
Methods
Younis et al. conducted a comprehensive literature search across PubMed, Web of Science, EBSCO, ClinicalTrials.gov, and the Cochrane Library for studies published from January 1978 to August 2019. Eligible studies examined the relationships between cycle length and ovarian reserve markers in women with regular cycles (21 to 35 days). Studies including participants with polycystic ovary syndrome (now polyendocrine metabolic ovarian syndrome), premature ovarian failure, oral contraceptive use, chemotherapy or radiotherapy exposure, and ovarian surgery were excluded to minimize confounders.
Cycle lengths were categorized as short (21 to 27 days), normal (28 to 31 days), long (32 to 35 days), or combined (28 to 35 days). The primary outcomes included ovarian reserve (AMH and AFC), natural fecundability, and IVF performance (number of retrieved oocytes and clinical pregnancy rates).
Results
Ovarian Reserve:
Three studies—which included 1,960 women—showed that women with short cycles had significantly lower AMH levels than those with normal cycles (P < 0.001). Interestingly, this decline was approximately eight times greater than the decrease expected from a one-year age difference. Similarly, three studies (n = 2,393) found significantly lower AFC among women with short cycles versus normal cycle lengths (P < 0.001), equivalent to approximately 11 to 13 years of natural ovarian aging.
Natural Fertility:
Four studies tracking 4,409 women over 19,979 cycles found that women with short cycles had reduced fecundability compared with those with normal cycles (odds ratio [OR] = 0.81; 95% CI: 0.72–0.91; P < 0.001), corresponding to about 19% lower odds of conception per cycle.
“Women with short cycles had reduced fecundability compared with those with normal cycles … corresponding to about 19% lower odds of conception per cycle.”
IVF Outcomes:
Three IVF studies including 5,392 women (7,697 cycles) demonstrated that women with short cycles produced fewer oocytes (P < 0.001). Five studies (n = 6,481; 8,802 cycles) found that shorter cycle length was associated with lower clinical pregnancy rates (OR = 0.76; 95% CI: 0.60–0.96; P = 0.02).
Discussion
These findings demonstrate that cycle length within the “normal” range does not translate to “normal” fertility potential. Women with 21 to 27 day cycles exhibited significantly lower ovarian reserve and poorer reproductive outcomes compared with those whose cycles lasted 28 to 31 days. This challenges the previously accepted assumption that any cycle length between 21 and 35 days reflects similar reproductive health.
From a FABM perspective, this study highlights a key distinction between ovulatory function and ovarian reserve. FABMs can identify the fertile window and confirm ovulation but cannot necessarily assess egg quantity or quality. Nonetheless, if women have consistently short cycles, even if they are regular, this may indicate a diminished ovarian reserve. Comprehensive fertility education should thus include awareness of age-related and individual variations in ovarian reserve.
Strengths and Limitations
This meta-analysis features a large, combined sample size, a systematic methodology, inclusion of both quantitative (AMH, AFC) and clinical (pregnancy) outcomes, and a low risk of bias. However, limitations include heterogeneity among studies, variability in AMH assays, and reliance on self-reported cycle lengths in some cohorts.
Implications
Overall female cycle length appears to reflect both quantitative (follicle number) and qualitative (fertility potential) dimensions of ovarian reserve, suggesting it may be a more comprehensive biomarker than tests such as AMH or AFC. Importantly, differences in ovarian reserve measures between women with short and normal cycle lengths were far greater than those attributable to age alone, indicating that cycle length identifies aspects of ovarian aging that age and AMH alone may miss.
“Overall female cycle length appears to reflect both quantitative (follicle number) and qualitative (fertility potential) dimensions of ovarian reserve, suggesting it may be a more comprehensive biomarker than tests such as AMH or AFC.”
Overall cycle length is a noninvasive, cost-free, and easily obtainable data point. Routine inquiries about average cycle length could serve as an early screening tool for identifying women at risk of diminished ovarian reserve. Those with consistently short cycles, especially women delaying childbearing, could be counseled about potential fertility implications and offered timely evaluation or treatment options.
Future Research
Further longitudinal research is needed to determine whether cycle length predicts time to menopause or the onset of premature ovarian insufficiency. Additional studies may also determine whether lifestyle or nutritional modifications can influence cycle length or slow ovarian aging. Research examining whether short cycles result from accelerated follicle recruitment, reduced inhibin B, or elevated FSH could suggest new etiologies of ovarian decline.
Conclusion
This systematic review and meta-analysis provide compelling evidence that ovulatory-menstrual cycle length within the normal range is strongly associated with ovarian reserve and fertility outcomes. Women with short cycles (21 to 27 days) exhibit significantly lower AMH and AFC values and reduced natural fecundability when compared with those whose cycles are 28 to 31 days long. These differences far exceed those predicted by age, suggesting cycle length may serve as an early indicator of ovarian aging.
In regard to fertility awareness and education, this research expands the conversation beyond ovulation detection. Regular cycles alone do not guarantee preserved fertility. Women already tracking their cycles through FABMs possess valuable longitudinal data, and integrating cycle length trends can encourage earlier, evidence-based reproductive decision-making and more personalized fertility counseling.
REFERENCES
[1] Younis, J. S., Radin, O., Izhaki, I., Haddad, S., Radin, E., & Bar-Ami, S. (2020). Association between menstrual cycle length and ovarian reserve: A systematic review and meta-analysis. Human Reproduction Update, 26 (4), 1–15. https://doi.org/10.1093/humupd/dmaa013
[2] Broekmans, F. J., Soules, M. R., & Fauser, B. C. (2009). Ovarian aging: Mechanisms and clinical consequences. Endocrine Reviews, 30 (5), 465–493. https://doi.org/10.1210/er.2009-0006
[3] Wallace, W. H., & Kelsey, T. W. (2010). Ovarian reserve and reproductive age may be determined from the number of non-growing follicles in the ovary. Human Reproduction, 25 (9), 2300–2308. https://doi.org/10.1093/humrep/deq220
[4] Broer, S. L., Broekmans, F. J., Laven, J. S., & Fauser, B. C. (2013). Anti-Müllerian hormone: Ovarian reserve testing and its potential clinical implications. Human Reproduction Update, 20 (5), 688–701. https://doi.org/10.1093/humupd/dmt038
[5] La Marca, A., & Volpe, A. (2006). Anti-Müllerian hormone (AMH) in female reproduction: Is measurement of circulating AMH a useful tool? Clinical Endocrinology, 64 (6), 603–610. https://doi.org/10.1111/j.1365-2265.2006.02533.
ABOUT THE AUTHOR
Lynnea Nicholls, DO is a Family Medicine resident at St. Anthony Family Medicine Center Orchard in Westminster, Colorado. She earned her Doctor of Osteopathic Medicine degree from Rocky Vista University College of Osteopathic Medicine in Parker, Colorado, after completing her undergraduate education at the University of California, Berkeley.
Dr. Nicholls has a particular interest in women’s health and preventive care. She completed the FACTS elective during her fourth year of medical school to deepen her understanding of fertility awareness-based methods and natural family planning, with the goal of equipping future patients to make informed decisions about their reproductive health.
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