The theory behind apirin is that, because it is a blood thinner, that it can prevent any clots in the placenta and improve the flow of cord blood thus preventing miscarriage and helping with blood pressure related issues such as pre eclampsia and HELLP. There has not been a heck of a lot of research in the pre natal area as yet, you will find most of the research is in other areas like general blood pressure studies etc where they know that x and y happen and therefore they say well if it worked there then maybe it will have the same effect here type thing.

I believe from what I have read of related research that it is a very promising idea and worth a shot IMHO