With my first (singleton), I ran (slowly!) into middle of 2nd tri. With the twins, I changed to walking by 8 weeks (and ended up on bed rest at 22 weeks...not related to running, but then I couldn't do anything).
I'm so hoping for a less remarkable journey this time. I've heard you can keep going at the level of training you were doing before becoming pregnant. Since my longest run is this weekend, I should be ok...I hope.