I have had three different consultants answer this question from me- all three said I must not take it until I get my BFP as before this it will make it LESS likely for implantation to occur.
If there is a clotting problem 75mg aspirin a day after BFP has been proven to reduce MC risk.
If there is no clotting problem 75mg aspirin a day after BFP has shown a slight MC reduction in some small studies, and no reduction in other studies. Often it is recomended on a 'it can't hurt/might help basis' in cases where there is no clear cause of MC.
The higher 150mg dose sometimes prescribed in clotting problem cases has a very unclear sucess rate- some studies show a slight help, most show no difference between 75 and 150.
My current consultant told me that as of today (last week) when i saw them, there is no convincing evidence that 150 is more helpful than 75, and there is less data proving it is safe.
Having said that, they then put me on 150! In cases like mine, and hairytale, where 75 has not worked, i think they try 150 just in case it does make a difference.
You can get cheap 75mg aspiring over the counter from any chemist.