Few Women Can Accurately Predict Day of Ovulation

Washington, DC—Only 1 in 8 women trying to conceive accurately identified their day of ovulation in a study presented at the 2011 meeting of the American College of Obstetricians and Gynecologists.

Jayne Ellis, PhD, of SPD Swiss Precision Diagnostics GmbH in Geneva, which makes the Clearblue Easy Fertility Monitor, presented the study.

“Women tend to think they ovulate around day 14, but there is a lot of variability in the menstrual cycle, and we found that many women ovulate much later,” she said in an interview. “Women who had previously used home ovulation tests were more likely to provide an answer to the question ‘When do you think you ovulate?,’ indicating that their use had provided some degree of education and awareness regarding their menstrual cycle. However, many women’s estimates were wrong for the cycle in which they conceived.”

The prospective conception study examined the accuracy of women’s estimation of the days they ovulate by comparing their estimated fertile days against an accurate analysis using urinary hormone monitoring. Only 56% of the women estimated correctly their ovulation time within a margin of ±2 days. This was likely a result of lack of awareness of their menstrual cycle characteristics and intracycle variability, according to Dr Ellis. During the study, women provided daily urine samples and used the Clearblue Fertility Monitor to aid in conception.

For each woman who became pregnant, a laboratory measurement of luteinizing hormone (LH) was used to verify LH surge in the cycle during which they conceived. Their ovulation day was calculated as the day after the confirmed LH surge.

Of the 130 women who became pregnant, 102 reported at recruitment that they knew their day of ovulation, including most of those who previously used an ovulation test. For women who reported a (perceived) day of ovulation, 51% believed that they ovulated on day 14 or 15; however, laboratory analysis showed that ovulation occurred on these 2 days in only 20% of the cycles that led to conception. The actual ovulation day ranged from day 9 to day 44.

The women’s perceptions and laboratory tests were an exact match in <13% of cases; they matched within 1 day in 38% of cases, and within 2 days in 56% of cases. The median difference in agreement was 2 days (ie, the calculated ovulation day was typically 2 days later than expected by the women); the range of inaccuracy spanned from –10 to +27 days.

For 22% of women, ovulation occurred earlier than predicted, and 44% were >2 days too late or 2 days too early in their predictions.

“If this latter group had been relying on calendar ovulation methods alone to predict peak fertility, they would have missed their opportunity to conceive,” Dr Ellis noted. “It would be expected that previous ovulation test users would have better awareness of their own cycle characteristics, but in our study, the numbers were too low to test this hypothesis. However, even if women are able to correctly cite their ‘average day of ovulation,’ since the typical menstrual cycle length variability is 6.7 days, it is not likely to be correct in any given cycle.”

“The study reinforces what is known —that women who are aware of their real fertile periods have much better chances of conceiving than those who just guess,” Dr Ellis commented.

She added that a prospective method for correctly identifying the fertile phase, which identifies the LH surge, is useful for couples who wish to time intercourse.