Running Safely During Pregnancy: A Runner's Guide

Question
Hi,
I have been a runner for several years. I've done many half marathons and did a full marathon last October. My last half marathon was late March of this year. Shortly after that race, I got pregnant. I am currently about 14 1/2 weeks pregnant. This is my first pregnancy. I've continued to run throughout the pregnancy, but have tapered off over the last couple of weeks from about 20-25 miles per week to about 10-12 miles per week. When I run, I haven't been pushing it, and really have been going pretty slow. At my first appointment, both the nurse and my OB said that it was okay to continue running because my body was used to it, but not to over-exert myself.

At my most recent appointment, I asked my doctor about the issue of not letting my body temperature get above 102 or 103 when running/exercising. She said she was less worried about that, and that I should try not to let my heartrate get over 140 when running. Honestly, I've never checked my heartrate during or after a run, so I have no idea what is normal for me. A week or so ago, I decided to check my heartrate after a short and slow 3 mile run. It was around 150 (assuming I took my pulse correctly). This really has me worried because I've run a fair amount during this pregnancy. Now I'm afraid that I could have harmed (or be harming) the fetus during my run.

I've read a lot of conflicting information online about heartrate guidelines, and I don't know what to believe. Several things that I've seen have said that the ACOG removed the heartrate limit of 140 a few years back.

I'd love to hear your opinion on this. The last thing I want to do is harm the fetus, so if I should stop running, I will. Fitness is important to me, though, so I'd like to stay active if possible. I'm just worried my heartrate will get up to 150 with any exercise.

Thanks for your input.

Kelly

Answer
ACOG lifted the advisement pregnant women keep their HRs below 140 BPM in 2002. The original study that prompted the HR restriction was very small, did not include athletic women, and was poorly designed.
Current guidelines advise that pregnant women use perceived exertion during aerobic exercise. Always stay in a zone that you feel is "somewhat difficult" and never exercise to the point of panting, or where you can not hold a normal conversation.
Since you are athletic, and well acclimated to running, I am sure that you did not harm your developing baby.
The placenta stops growing at 20 weeks. So from this point onward, its ability to transport oxygen is capped. In the last half of pregnancy, your baby grows rapidly, which increases her demand for oxygen. So your rate of perceived exertion will rise accordingly, and you will need to taper the intensity of your workouts to stay within the proper training zone for pregnancy.
Some women can run/jog safely throughout pregnancy, but most find that brisk fitness walking provides the proper amount of exertion during the last trimester.
Because you engage in high impact activity, you will need monitor your joints, especially the ankles, knees, pubus, and sacroiliac. Pregnancy hormones lengthen ligaments and other connective tissues, so that that pelvic outlet can widen during delivery.
If you develop pain or discomfort in your joints, you should stop all high impact activity. Running on concrete, or downhill, places additional stress on your joints and is not advisable during pregnancy.