Determining the Fertile Phase: A Comnparison of the Clearplan Easy Fertility Monitor(TM) With Self-Assessment of Cervical Mucus
Fertility Monitors

Richard J. Fehring
Reprint from Current Medical Research
Vol 15, No 1-2, Winter/Spring 2004
Washington, DC
Reproduced with Permission

The accurate determination of the 6 days of fertility during the menstrual cycle (i.e., the fertile phase) and the peak of fertility is critical for women and couples who wish to use natural biological markers to achieve or avoid pregnancy. Self-monitoring of cervical-vaginal mucus is one of the most common biological markers used in the selfassessment of the fertile window. However, cervical mucus can at times be a confusing and subjective biological marker.1,2,3,4 The Clearplan Easy Fertility Monitor (CPEFM) (Unipath Diagnostics Company, Princeton, New Jersey) is a relatively new electronic device that has been developed to help women determine their fertile window.5 The CPEFM measures urinary metabolites of estrogen and LH and provides the user with a daily indication of "low", "high" and "peak" fertility. Evidence from the manufacturer shows that, on average, the monitor will provide the user with 3-5 days of high to peak fertility. Research also shows a strong correlation of the CPEFM "peak" with the actual day of ovulation as determined by serial transvaginal ultrasound and serum measures of LH and the rise of circulating estrogen.6

Researchers from Marquette University and Atlanta, Georgia recently conducted a study to compare the beginning, peak, and length of fertility in a woman's menstrual cycle as determined by the CPEFM with the beginning, peak, and length of the fertile period as determined by self-monitoring of cervical mucus.7 The comparison was made with a 6-day fertile phase as the standard for comparison (i.e., the day of ovulation and the 5 days preceding ovulation). The estimated day of ovulation was defined as the day after the LH surge or, i.e., the second "peak" day on the CPEFM. One hundred volunteer women participants (mean age 29.4) were recruited for this study. They observed their cervical mucus and monitored their urine for estrogen and LH metabolites with the CPEFM on a daily basis for 2-6 cycles and generated 378 cycles of data; of these, 347 (92%) had a CPEFM peak. The beginning of the fertile window was, on average, day 11.8 (SD = 3.4) by the monitor and day 9.9 (SD = 3.0) by cervical mucus (r = 0.43, p < 0.001). The average first day of peak fertility by the monitor was 16.5 (SD = 3.6) and by cervical mucus 16.3 (SD = 3.7) (r = 0.85, p < 0.001). The mean length of the fertile phase by the monitor was 7.7 days (SD = 3.1) and by cervical mucus 10.9 days (SD = 3.7) (t = 12.7, p < 0.001).

Therefore, the beginning of the ferdle phase as determined by the CPEFM was, on average, almost 2 days later than the beginning of fertile phase as determined by the selfobservation of cervical mucus. These results make sense in that the monitor is based on an elevated threshold level of urinary estrogen and is designed to target the high and peak days of fertility, i.e., narrow the fertile window. The first externally observed signs of cervical mucus probably reflect a lower level of circulating estrogen.

The average day for the peak of fer!blity by the CPEFM and the self-observation of cervical mucus were very similar (i.e., day 16). The fact that the average length of the fertile phase as determined by cervical mucus was statistically longer by almost 4 days as compared to the CPEFM fertile phase was not surprising. The fertility monitor was designed to narrow the fertile window and past studies have shown that cervical mucus observations overestimate the fertile window by almost 200%. Cervical mucus monitoring also underestimated the beginning of the estimated 6-day fer!dle phase in about 26% of the cycles in this study. However, as expected, the CPEFM misses the beginning of the fertile window about 40% of the time.


Use of the monitor as a sole source of avoiding pregnancy would be easier and more accurate if the monitor provided a longer pre-ovulatory warning of fertility, i.e., detected a lower threshold of estrogen. On average, the monitor provides a long enough warning time in about 60% of the cycles. Therefore, to use the monitor as a method to avoid pregnancy would require the use of another marker, unless the woman user is comfortable with a 60% accuracy for the beginning of the fertile phase or uses the monitor as a post-ovulatory device only. Studies are underway to determine the effectiveness of these 2 approaches in using the CPEFM monitor as an aid to estimating the fertile window and avoiding pregnancy.(Richard J. Fehring)


1 Guida, M., Tommaselli, G.A., Palomba, S., et al. Efficacy of methods for determining ovulation in a natural family planning program. Fertility & Sterility. 1999;72:900-4. [Back]

2 Fehring, R. Accuracy of the peak day of cervical mucus as a biological marker of fertility. Contraception. 2002;66:231-5. [Back]

3 Moghissi, K.S. Prediction and detection of ovulation. Fertility & Sterility. 1980;34:89-98. [Back]

4 Moghissi, K.S. Cervical mucus changes and ovulation prediction and detection. Journal of Reproductive Medicine. 1986;31(Suppl): 748-53. [Back]

5 May, K. Home monitoring with the ClearPlan Easy Fertility Monitor for fertility awareness. Journal of International Medical Research. 2001;29 (Suppl 1): 14A-20A. [Back]

6 Behre, H.M., Kublage, J., Gassner, C., et al. Prediction of ovulation by urinary homone measurements with the home use Clearplan Fertilib Monitor: comparison with transvaginal ultrasound scans and serum hormone measurements. Human Reproduction. 2000;15:2478-82. [Back]

7 Fehring, R., Raviele, K. and Schneider, M. A comparison of the fertile phase as determined by the Clearplan Easy Fertilib Monitor and self-assessment of cervical mucus. Contraception. 2004;69:9-14. [Back]