Once you know the fertile window is only about six days long, the next question is practical: how do you actually detect it at home? The right tool depends on whether your cycles are regular or not.
Regular cycles
With regular cycles, urine LH tests or basal body temperature can work well. Data is collected over about 3 months by a companion app that then predicts the fertile phase. Because the prediction is based on past cycles rather than live hormone changes, any variation this month can throw it off.
Irregular cycles
About one in three women has irregular cycles. In that case you rely on your body's signs or on more expensive tests that detect the rise in estrogen (or the drop in FSH) a few days before ovulation. Increasingly, women — even those with regular cycles — learn the natural signs of their fertility. Babyndex complements this by reading the crystals in dry saliva samples during the fertile period.
Whichever method you pick, the goal is the same: catch the days before ovulation, not the day of ovulation itself. That is when conception is most likely.