Some doctors will say yes, it's safe, some will say that it isn't.
But in reality it all depends on you. Do you see yourself as weak? Is your body strong?
If you don't see yourself as weak, and have a low body weight, and very low stomach/back fat, then sure, go right ahead, you'll feel your limits. If there comes a point when you feel as though it's affecting your womb, than stop.
If you see yourself as weak, or as having stomach/back fat, then I suggest you seriously don't. Although the fat may seem like it's protecting the baby, it's really harming it. Why? Because your fat will move rapidly up and down as you're running, and while that's good exersize, it will seriously affect the womb.
There are other ways to exersize, you can do light yoga, or brisk walking, or areobics.