Accuracy of perception of ovulation day in women trying to conceive.

Background: The likelihood of conception is increased if intercourse is timed to coincide with the fertile period (5 days up to ovulation). However, to be effective, this requires good awareness of the day of ovulation. The aim of this study was to examine the accuracy of women's perceived ovulation day, compared with actual fertile days, in a cohort of women trying to conceive.

Main Outcome Measures: Comparison of women's estimated day of ovulation with their actual ovulation day (determined by detecting luteinising hormone).

Methods: This was a sample collection study and volunteer women were recruited via online advertising. At recruitment volunteers reported the cycle day they believed they ovulated. They then used a home urine fertility monitor to test their daily fertility status to time intercourse to try and achieve conception, in addition to collecting early morning urine samples for laboratory analysis. The main outcome measure was a comparison of women's estimated day of ovulation with their actual ovulation day, as determined by urine detection of luteinising hormone.

Results: Three hundred and thirty women were recruited onto the study and data was available for 102 volunteers who became pregnant. Thirteen women (12.7%) correctly estimated their ovulation day; median difference +2 days, range -10 to +27 days. The most common days for estimation of ovulation were day 14 (35.5%) and day 15 (15.7%). Only 55% of estimated ovulation days fell within the volunteers' fertile window; only 27% on days of peak fertility.

Conclusions: Women trying to conceive may benefit from using a prospective method to identify their fertile phase, as a significant proportion could be incorrectly estimating their fertile days. These observations were made on women who were actively looking for knowledge on fertility and considered only cycles where conception occurred, inaccuracy could be greater if a broader population is considered.