Determining exactly when ovulation occurs remains difficult for women to do on their own. Of two widely available methods, one overestimates, while the other underestimates the actual window of fertility, according to a recently published study.
Researchers compared the beginning, peak, and length of the fertile period in a woman's menstrual cycle as determined by the Clearplan Easy Fertility Monitor (CPEFM) with the beginning, peak, and length of the fertile period as determined by self-monitoring of cervical mucus. The CPEFM measures urinary metabolites of estrogen and luteinizing hormone and provides the user with a daily indication of "low," "high," and "peak" fertility.
The beginning of the fertile window was, on average, day 11.8 with the monitor and day 9.9 with mucus monitoring. The average first day of peak fertility was day 16.5 with the monitor and day 16.3 with mucus monitoring. And the mean length of the fertile window was 7.7 days with the monitor and 10.9 days with self-monitoring.
The bottom line is that either method is appropriate for couples trying to conceive, but the investigators stopped short of recommending the test kit, which may underestimate the length of the fertile window, to help a couple avoid pregnancy.
Fehring RJ, Raviele K, Schneider M. A comparison of the fertile phase as determined by the Clearplan Easy Fertility Monitor and self-assessment of cervical mucus. Contraception. 2004;69:9-14.