Why did femSense show a different ovulation date to my OPK?

Ovulation tests are used to determine when you ovulate and are most likely to get pregnant. Since the different methods of ovulation detection measure different bodily signals, they usually are “positive” on different days. While ovulation test strips measure the hormone that shows that the body is most likely preparing to ovulate, femSense measures your body temperature continuously to detect the temperature rise that happens after ovulation. The femSense app can predict the start of your fertile window and the femSense sensor patch can then measure and confirm ovulation.

In short, ovulation tests confirm that your body is preparing for ovulation. femSense confirms your fertile days and that an ovulation did occur.