I've watched a number of my former teammates as they've had their first children. Most of them continued to run at least occasionally through their pregnancies. Some of them tried to stay "fit" through their pregnancies so they could come back afterwards and be competitive as soon as possible. One of these friends had already qualified for the upcoming marathon OTs, so she wanted to stay as fit as possible to be able to get back into her competitive fitness asap post partem.
What I would say observing these friends of mine is: don't try to stay fit during pregnancy. Rather, just run what feels comfortable as part of remaining healthy. Trying to stay race fit is an awful idea. My friend is now struggling 4 or 5 months after giving birth. I'm convinced she would have been better off scaling back her running a lot more, especially in the third trimester. She would have come through the pregnancy much healthier overall and less physically stressed, so that would have allowed her to get back into runnning hard sooner.
When you're pregnant, your body is prioritizing creating this new life. If you're taxing yourself by trying to work out too hard, its your body that is going to suffer, more so than the developing baby. Those injuries you develop while pregnant (which you might not even notice until after you've given birth and started training hard again) will stick with you and could end your running career.
My advice would be to taper your running down both in volume and intensity over the pregnancy until by the third trimester or so, you are doing only a few days a week of some easy shorter runs, as feels comfortable, with some non impact exercise such as swimming or elliptical to augment a little if you want. In that third trimester, further taper over to only non-impact exercise.
Lastly, don't feel like what others have been able to do in their pregnancy in any way means that you should also be able to do that. My friend was running 7 or 8 mile runs in her third trimester. If you have a friend like that, it will be easy for you to feel like you "should" be able to do that too. Don't feel this way. Not only is everybody different, but just because your friend advertised on facebook all the running she was doing while pregnant doesn't mean it won't have a long term negative impact on her (as was the case with my friend). Chances are, she won't be advertising that aspect on facebook.
Good luck and congrats