March 18, 2024

Fertile Phase Variability in the Menstrual Cycle: A Review of Research

By: Seth Chauhan, MD

Director’s Note: This week we feature research summarized by Dr. Seth Chauhan, a first-year family medicine resident and former FACTS elective participant. Previous studies have established a standard physiologic fertile window of up to 6 to 7 days during which a woman can conceive. The study by Fehring and Schneider [1] reviewed below explores the potential window of fertility as determined by cervical mucus and/or urinary hormone observations. In this study,  “Variability in the Hormonally Estimated Fertile Phase of the Menstrual Cycle,” the authors sought to determine the extent of variation in the potential days of fertility. The results can guide couples to identify their fertile window in which to avoid sexual relations to prevent pregnancy.

 

Introduction

The “6-day fertile window” has traditionally been the standard for women with regular cycle length. Yet, studies show the fertile window is more variable than what was previously assumed. A study by Keulers et al measured the fertile window by analyzing sperm-mucus interaction in the laboratory alongside serial ultrasound to detect follicular rupture. [2] The study by Fehring and Schneider summarized below seeks to further investigate the variability of the fertility window in healthy women by using a combination of mucus changes and hormone monitoring methods. [1]

 Methodology

This study used a retrospective analysis of 1,060 menstrual cycles from 165 women with normal cycles between 21 and 42 days. The participants were selected from women seeking instruction in natural methods of family planning and were selected from three cities in the United States (Atlanta, Milwaukee, St. Louis). The average age of women enrolled in the study was 29 years old.

Participants were taught to chart their cervical mucus on a scale of 1 to 4. The first high mucus rating > 2 indicated the start of the fertile window. In addition to mucus charting, participants measured urinary metabolites of estrogen and luteinizing hormone (LH) using the Clearblue Easy Fertility Monitor (CEFM). The first reading of “High” on the monitor indicated the start of the fertile period corresponding to threshold levels of estrone-3-glucuronide (E3G). The last day of the fertile window calculating the Estimated Date of Ovulation (EDO) was indicated by the second “Peak” day of the monitor, which indicates threshold reading of LH.

The following inclusion criteria were used:

  1. The woman needed to have ≥ 3 menstrual cycles
  2. The CEFM had to clearly identify a peak indicating ovulation

Results

Each participant provided a mean of 5.2 cycles for data with a mean menstrual cycle length of 28.97 days. When using only the CEFM to measure E3G threshold levels, the mean fertile window was 5.1 days with the most frequent length at 3 days (21.4%) and a range from < 1 to > 7 days, with 16.1% measuring ≥ 7 days. This data closely matched the results of Kuelers et al, who found the length of the fertile window ranged from 1 to 5 days with a most frequent duration of 3 days. [2]

When participants used a combination of cervical mucus as the starting point and the CEFM detection of ovulation as the end point of the fertile window, the mean fertile window length increased 6.6 days. When the length of the potential fertile window was based solely on cervical mucus measurements, the mean duration of the fertile window increased to 10.49 days. Of the cycles measured with cervical mucus charting, 46% were ≥ 7 days.

“When using only the CEFM to measure E3G threshold levels, the mean fertile window was 5.1 days … When the length of the potential fertile window was based solely on cervical mucus measurements, the mean duration of the fertile window increased to 10.49 days.”

The frequency of intradifference in fertility length ≥ 7 days was 23.4%, and the mean intravariability was 5.14 days. It is unclear from the study whether these data indicate the variability of fertile window length of different cycles from the same woman or some other measurement.

Happy woman studying online, watching webinar, podcast on laptop, making notes. E-learning concept.

Discussion

The purpose of this study was to further investigate the variability in the hormonally estimated fertile phase of the menstrual cycle. The study reviewed the cycles of 140 participants with regular cycles using electronic hormonal fertility monitors as well as mucus-based charting.

When using CEFM only, the mean fertility window was 5.1 days with a range from < 1 to > 7 days. When using only the high-rated cervical mucus as an indicator of fertility, the mean potential fertility window increased to 10.4 days. This wide variability between methods suggests that a rise in urinary estradiol is a more accurate measure of the start of the fertile window than cervical mucus changes.

“This wide variability between methods suggests that a rise in urinary estradiol is a more accurate measure of the start of the fertile window than cervical mucus changes.”

 

An important distinction in this study is that the women were trained to use a scale of 1 to 4 to rate their cervical mucus (with ratings of 3 or 4 indicating the fertile window) rather than a particular method such as the Billings Ovulation Method or the Creighton Model. Therefore, the resulting differences cannot be extrapolated to these methods; further research must be done to accurately compare the differences in measuring the fertile window.

Additionally, the data suggest the traditional standard of the 6-day fertility window is not accurate when accounting for the wide variety of fertility length among participants. Although the average length of fertile days among participants was 5 days, only 11.8% of cycles had a fertility window of exactly 5 days. The most common fertile window was 3 days, which accounted for only 21% of cycles. The wide variability of fertile days measured in this study underscores the importance of using fertility awareness models to accurately discover each woman’s fertile window rather than assuming the standard 6 days of fertility.

This study builds upon the findings from Keulers et al, who used sperm-to-mucus interaction and serial ultrasounds to measure the fertile window; they found similar variability from < 1 to > 5 days as well as a strong association between longer fertile window length and rate of conception. [2] While this study did not measure the rate of conception in relation to the fertile window length, this would be an important area of future study, especially investigating how intercourse at different points in the fertile window may affect pregnancy outcomes.

A limitation of this study is the length of the fertile window was measured using LH peak values rather than the gold standard of serial ultrasounds to confirm follicular rupture. However, peak LH has a 98% accuracy of measuring ovulation when measured in conjunction with serial ultrasounds of the ovaries and is thus considered an acceptable and more convenient marker of ovulation. [3]

 “This study by Fehring and Schneider further established that variability of the fertile window exists between different women and even from cycle to cycle in an individual woman. New technologies like CEFM … may help couples accurately determine their fertile window to achieve their pregnancy goals.”

In conclusion, this study by Fehring and Schneider further established that variability of the fertile window exists between different women and even from cycle to cycle in an individual woman. New technologies like CEFM, which are becoming more available and affordable, may help couples accurately determine their fertile window to achieve their pregnancy goals.

 

References

[1] Fehring RJ, Schneider M. Variability in the hormonally estimated fertile phase of the menstrual cycle. Fertil Steril. 2008 Oct;90(4):1232-5. doi: 10.1016/j.fertnstert.2007.10.050. Epub 2008 Feb 4. PMID: 18249381.​
[2] Keulers MJ, Hamilton CJ, Franx A, Evers JL, Bots RS. The length of the fertile window is associated with the chance of spontaneously conceiving an ongoing pregnancy in subfertile couples. Hum Reprod. 2007 Jun;22(6):1652-6. doi: 10.1093/humrep/dem051. Epub 2007 Apr 20. PMID: 17449509.
[3] Behre HM, Kuhlage J, Gassner C, et al. Prediction of ovulation by urinary hormone measurements with the home use Clearplan Fertility Monitor: comparison with transvaginal ultrasound scans and serum hormone measurements. Hum Reprod 2000;15:2478–82.

ABOUT THE AUTHOR

Seth Chauhan, MD

Seth Chauhan, MD is a first-year family medicine resident at Via Christi in Wichita, KS. He completed medical school at Louisiana State University School of Medicine and his undergraduate education at The University of Alabama in Tuscaloosa, AL. Dr. Chauhan is interested in underserved medicine and women’s health. He enrolled in the FACTS elective to gain a better understanding of natural family planning methods and ways to share these methods with future patients so they feel more empowered over their health and reproductive decisions.

Pin It on Pinterest