The last two times, they wanted to wait and see a heartbeat before
starting the blood thinners.
I'm now 31 weeks and will continue Lovenox
until I get a little closer to delivery, then switch to heparin, since it has a shorter half life and they don't want you to be on blood thinners the day of delivery. Of course, I will never know whether the reason this pregnancy continued was due to the Lovenox or just a conicidence...but I was not taking any chances this time, and I'm glad I didn't.