At 5 weeks, my doctor recommended I start taking 81mg of aspirin. At that appointment I had an ultrasound and everything with the fetus was normal - normal size, development, etc. A week and a half after starting the aspirin, I started to spot and eventually had a miscarriage. I had a few ultrasounds while I was spotting and we found out that the fetus had stopped growing at 5.5-6 weeks - right around the time I started the aspirin. The hard part was, the heart was still beating towards the end and there was nothing we could do except wait for the inevitable. I miscarried at 9, almost 10 weeks.
When I went for my 6 week checkup after the miscarriage, I saw another doctor in the practice and he was shocked that the first doctor started the aspirin so early. He said that if you're not already on aspirin prior to conception, they wait to have you start taking aspirin until week 11 or 12 - after the placenta has had a chance to attach itself successfully. I asked him if he thought the aspirin led to the miscarriage. He said that there's no way of knowing what actually led to the miscarriage, but that the aspirin certainly didn't help.
This is just my experience as well. I don't have chronic high blood pressure and all of my blood work came back normal (for clotting disorders, etc). You might want to ask your doctor what her opinion is on starting aspirin toward the end of the first trimester. I actually think that's when the studies that have involved taking low dose aspirin, start anyway (around week 12, but before week 14).
Good luck with your pregnancy.