L.Z.
Baby aspirin doesn't increase fertility per se, it increases blood flow and prevents clotting, so the idea behind its connection to fertility is assisting with implantation of the embryo to your uterus. So, in theory, if you were trying to get pregnant on your fertile days, once you enter the luteal phase and ovulation has occurred, you take a baby aspirin once a day until (and if) you get your period. I was actually just told this very thing yesterday by a reproductive endocrinologist at a fertility center!
For what it's worth, you can learn more about ovulation and the luteal phase, etc. by reading "Taking Charge of your Fertility" by Toni Weschler. I'm not saying you haven't tried this already, or that you don't know what you are doing, but it can teach you more about your body and how it works in general, so some of these questions can be more logically understood. It taught me so much simply about womanhood!