Recently, I wrote an article which five simple lifestyle interventions would bring immediate and measurable benefits to women over 35 in terms of fertility. However, if I had to pick just a single one that would be:

At your next annual checkup ask your gynecologist and ask him to check your FSH and AMH levels!

These two simple values give an excellent estimate of how many eggs you have left. If you schedule your visit to the doctor at the beginning of your cycle (day 1-5) you can have both readouts done from only one single blood draw (while AMH level is stable and can be checked every day, FSH oscillates a lot and tends to be highest and the beginning of each menstrual cycle).

Also talk to your mom to see if there was any unusual fertility history in the family (did one of your female relatives enter menopause too early, for example). And please, please avoid fertility apps and similar technological and medical over-kills! These things are helpful only to those who are earning money with them and have nothing to do with you connecting to your body and maximizing your natural fertility! Therefore, instead of putting your trust into apps, trust your body and learn to read the signs which it sends to you every day. This way you will best know your fertility status.