First of all, congratulations, that's wonderful news! Second, I'm not a doctor, so I can only speak from my experience and from what I've read, ultimately you should talk to your doctor and figure out what is best for you. Anyway, I haven't adhered to the 140bpm since I found out I was pregnant. I, along with my doctor, decided that perceived rate of exertion was the best route to go and is what ACOG is actually now trending towards. Basically, you have to be able to hold a conversation or say an 8 word sentence without being out of breath at the end. I would HIGHLY suggest Dr. James Clapp's book, Exercising Through Your Pregnancy. I found that it was incredibly helpful and I read that, probably as soon as I got pregnant, and then had a discussion with my doctor about it. I definitley backed off my intensity right away, but pretty much kept up the mileage until about 20 weeks and then started backing off. I think my longest runs up to 20 weeks were about 7 or 8 miles and then after that I backed off to 5 miles and then 4 miles and now I'm 36 weeks pregnant and just chugging along doing my 3 miles more so because I'm just feeling heavy and not as comfortable after the run if I do more. Even though I don't adhere to the heart rate thing, I always wear a heart rate monitor while I run. I just kind of like to know the range I'm in (but I did that before I was pregnant) and I just think it helps me gauge where I'm at a little better. While running my heart rate is almost always over 140 and I'm fine with that and so is my doctor because I'm running at a pace where I can have a conversation and I'm not huffing and puffing. One of the big things my doctor told me to be concerned about is not overheating or getting hot in general as you can sweat to cool yourself off, but the baby can't so you don't want to cook the baby! The other big thing is remaining hydrated. I have noticed that I drink a lot more water, especially while working out, since I've been pregnant. So, although I used to not want/need water for a 6 mile run, since I've been pregnant I definitely need water on a 6 miler. It is really important throughout your pregnancy, both working out and not, to remain hydrated, so just keep that in mind. In regards to pace, I've just gotten progressively slower! At 36 weeks, I'm kind of like a snail! Ha-ha! To give you an idea, prior to getting pregnant I was running about 8 min miles. Once I got pregnant, I slowed down to about 9 or so min miles. As I got further along and as I started to gain weight and as I started to show, I just got progressively slower. The weight slows me down, plus my stride has changed due to a sack of potatoes being strapped to my front! So, now I'm probably at a 10:30 min mile. Just go with how you feel (obviously after talking to your doctor) and I have kind of figured out that my body will let me know if I'm pushing a bit. The one thing I will say is that pregnancy is not the time to ignore pains while running. While I used to run through aches and pains, my doctor warned me now is not the time to do that. So, if I get uncomfortable or feel any pains, I just stop and walk. Oh and kind of obvious, but if you ever have bleeding stop running.
Good luck to you! Check with your doctor about it all, but a lot of doctors now seem to be leaning towards perceived rate of exertion. Feel free to PM me or ask me any other questions as I'm happy to give you what my experience has been throughout my pregnancy! Also, you are warned, get ready (once you start showing) to get a lot of nasty looks from people and really uncalled for comments from people if you are continuing to run and workout! I've had complete strangers tell me I'm hurting my baby by running! People can really be so ignorant!