It has been an interesting year for me as a personal trainer. Both of my daughters, and four of my clients became pregnant. They were all wondering if strength training during pregnancy is safe, and just as importantly, effective for weight maintenance and a healthy baby. There is no doubt in my mind that strength training is safe and effective. Before I worked with my pregnant clients, my husband trained pregnant women with his strength training program in the past. The most interesting outcome from that is the women got back into their pre-pregnancy shape within 6 months to a year, where most women who did not strength train struggled with getting their body back for years. My first pregnant client (this was 3 years ago), noticed that she had more energy and was stronger throughout her second pregnancy (I started training her after her first child), and she was able to work out during her whole pregnancy. With her first pregnancy, she did not work out at all because of the advice she got from family, and ironically, other fitness professionals. She was still a bit tepid about exercising with me during her pregnancy, but she did it. After she had her baby, she got her lean, youthful body back after 8 months! She also got pregnant again, and this time she is much more comfortable about the process. She knows the baby will be fine...and she knows she will get her body back very fast. So yes, strength training is effective, and I highly recommend it.
You do need to be cautious and selective about the exercises you do while you are pregnant. For example, it is not a good idea to do any exercises where you are laying on your back, for this makes it very difficult for the baby to move into position for the birth. My daughter, Nicole has this exact problem. She did not exercise as I had recommended (she lives in another state, so she did not see me), and she would sit in a prone position for long periods when she got home. She is close to the end of her pregnancy, and the baby is in a posterior position, with the head not lined up right for delivery. Moreover, it is very difficult for the baby to turn now that he is so big (more on this later). You also want to gauge the intensity of your workouts as well. As a simple rule, train with good intensity during the first trimester, and slowly back off the intensity the further into your pregnancy you get. Never over do it, but do continue all the way until the birth. Your body will have good muscle memory, and it will bounce back much faster if you do this.
As for Nicole, I offered her more advice as to how she can help get the baby into position. I found a website that demonstrated exercises she can do with a fit ball to help the baby. You can find out more about it here. Even if she would just sit on the fit ball, for example, more often, it can help the baby get repositioned.
Of course, in my opinion, it would be much better for women to start an exercise program before they get pregnant, but that should not be a barrier to start a strength training program while pregnant, You can, but you must be cautious. Find a good, experienced fitness professional to help you. My husband and I started an online instructional fitness program that is helpful too. In it, you can even communicate with us through our forum to help you through the process. You can find it here at HYSTRENGTH.
You do want to discuss your fitness program with your doctor or midwife as well. They can offer good tips on how to insure you do not over do it. But once again, do not be afraid of doing some strength training while pregnant.
To summarize, strength training is safe to do while you are pregnant. Furthermore, I think the benefits of strength training while pregnant are incredible. You will have more energy and strength to get through the day, and you will get your body back fast. You can't beat that.