There was a study published about 13 years ago in which women undergoing IVF were randomly assigned to either low-dose aspirin or placebo during ovarian stimulation. The women receiving aspirin (it was 100 mg/day in the original study) had a better ovarian response (with almost twice as many eggs obtained in the treatment group) and significantly higher implantation and pregnancy rates.
When I first heard the findings presented at a fertility meeting (in Tours, France; ah, those were the days!), I was impressed - here is an inexpensive medicine that almost doubles the IVF pregnancy rate. The study seemed well designed and the results clear (but in reviewing the paper I see that although 298 patients were randomized, Table 1 in the manuscript reports the results on only 74 women). Here is the citation if you want to dig up the article yourself: Rubinstein M, Marazzi A, de Fried EP. Low-dose aspirin treatment improves ovarian responsiveness, uterine and ovarian blood flow velocity, implantation, and pregnancy rates in patients undergoing in vitro fertilization: a prospective, randomized, double-blind placebo-controlled assay. Fertility and Sterility 1999;71(5):825-829. The authors theorized that aspirin improved the blood flow to the ovaries and uterus, which led to the beneficial effects.
I suspect that within a year of this article being published, more than half the IVF patients in the US were on low-dose aspirin. Since then, at least 12 randomized controlled trials of aspirin treatment during IVF have been performed, and the conclusion is ... aspirin does nothing to improve the success of IVF. Here is a recent meta-analysis of all the studies: http://www2.cochrane.org/reviews/en/ab004832.html
It's too bad, really. It was such a nice story.