hello turtle!
welcome and congratulations!
yes - you will slow. it's normal to do so, and in fact, it's very important that you don't overdo it. your body will tell you when you are working too hard, and in my experience, if i get carried away with my running and 'forget' to be careful, i end up feeling very very sick quite quickly!
one tip: try leaving your watch at home, or don't time yourself. just enjoy being able to run and relish the thought of the good it is doing baby and you. several of my friends COULDN'T do anything during pregnancy because of high blood pressure and bleeding (i think hegs also had problems from quite early on, so she can confirm how frustrating that is). it's FANTASTIC that you are still out there running. make the most of that. and forget about times!
don't forget that your body is working hard making the baby and all the other bits the baby needs. your resting heart rate will increase during pregnancy, and your heart is working harder to pump extra blood around the body to support the growing baby. my resting HR has gone up by about 10 bpm already (I'm 18 weeks now).
the most important thing is to listen to your body, and keep exercising at a comfortable level for as long as you can. if you have to stop running because of pain, discomfort or because you are worried about anything, that's absolutely fine too! and if you can do some swimming, x-training etc as well as running, that may give you something to carry on with once you do stop running.
i'm just taking it a run at a time at the moment. i'd love to be running throughout my pregnancy, and i'm totally NOT bothered about how slow i become. i ran an 8mile race at the weekend, and started right at the back so as not to get carried away. i didn't finish last, but was sooooo much slower than i normally would be. it really doesn't matter, though. i was plodding along on sunday grinning stupidly to myself because it was a sunny, cold day, and there i was, bringing up the rear of the field but feeling fantastic because i was running. and because i could run. and so many people can't. and i was enjoying myself, because i know it's good for me and baby.
and as for getting back into it after pregnancy, as clare said, many people find that they are actually quite fit after pregnancy. being pregnant is hard work for your body with the extra weight and functions it has to perform. and if you continue to do some exercise throughout, your body will be in the best possible shape to pick it up again once baby is born and you are given the all clear to get back out there again.
and if you have to stop completely, then so be it. your baby and your health are more important than how fast you can run, after all!
i too am quite scared at the thought of having to start from scratch once the baby is born, particularly because mine is due in july, and i am deferring my FLM place to 07, so i will need to get out again fairly soon to start building up the miles for the marathon. but if i have to do a run/walk thing and get around in 5 hours, i will!
hegs will be able to comment on this if she is around, as she is training for FLM now and had a baby last year...
sorry that was a bit of an essay. ;-)