Running while being pregnant is not good for you or the baby, period. I don't care where you heard that it was ok, because it is not and here are the reasons why:
Oxygen. The more oxygen that you need to use while exercising, the less oxygen is left for your fetus. While being pregnant you will never want to go over the anaerobic threshold, where the body can no longer produce enough energy for the muscles with normal oxygen intake. You will always want to stay in the 40-50% of max heart rate to assure your fetus is getting the right amount of oxygen. You often hear that your pulse should never go above 140 bpm, and that is very true. But focus needs to also be put on the fact that you should never go above 50% of max heart rate. 100% of max heart rate is calculated 220- your age.
Balance. When you are pregnant your balance is thrown off because your abs are being divided (your core is the place where your center of gravity is located and where all movement begins), and it's easy to fall or lose control while running.
Arched back. All pregnant women get an arched back and that is because the core stabilizers cannot stabilize your body properly anymore, again because the abs are being divided. This causes a lot of extra stress on the spine which is now overactive, running in this state will cause even more stress on the back and could lead to some serious back problems for the rest of your life.
Posture. You posture will change while being pregnant, see the above statement and also the pressure of the uterus. The pressure of the uterus will make it harder for the blood to circulate to and back from the legs, this is why some pregnant women get blood clots. As you use your legs to run this will cause extra stress on the lower extremity blood flow and cause too much stress on the body leading to injury and this will also affect the blood flow to your uterus. You will also want to avoid the jumping movement that is associated with running though the pressure on the pelvic area will be too stressful and the extra weight from fetus/uterus can cause hip problems.
Heat. As a pregnant women you are never allowed to get too warm, which comes automatically with running, especially when you go above the aerobic threshold.
I have a great pulse and could easily run while being pregnant and I would not get up to 140 bmp because I was super fit before getting pregnant. But I would never ever risk my baby's health just for me to burn some extra calories and have something to brag about later. My baby girl is my world and I'm only exercising in a way that I know is safe for her and will benefit her. I cant believe when I hear that pregnant women are running, that is such a selfish act and it really gets me mad. I know that doctors sometimes say that it is ok to run, but they are not certified personal trainers and has had a different education. They are experts in delivering your child, not exercise. And some pregnancy sites also state that it is ok, but the people who wrote that aren't certified in anything, all they did was start a site! I'm so tired of people who are not certified talking out of their butts. Do long walks, lift light weights, swim, don't eat like a pig, drink water and wont gain that much weight to begin with and it will get easier to get rid of. And never run while you are pregnant. Save that for when you are back to only being one person again .