Fertility monitors, OPK instructions, and most books & websites say that the day of ovulation is the most fertile day. I was reminded of this today when, just for fun, I took this quiz on Baby Center. If you answer “one or two days before ovulation” to question 3, it will tell you you’re wrong and that it’s the day of ovulation.
Except the quiz is wrong. See, for example, this study, just one among many that finds that the day of ovulation isn’t all that fertile (Figure 1 is the best illustration). One or two days before is better. The one study that did find the highest fertility on the day of ovulation was later updated to eliminate early miscarriages. When this is done, the day before ovulation was the most fertile even in that study.
In The Impatient Woman’s Guide, I describe how to modify charting, the fertility monitor, and OPKs to take advantage of this fact. It’s important, because if you use these methods and assume the day of ovulation is the most fertile you might be decreasing instead of increasing your chances of getting pregnant.