In many cultures, expectant moms are told to take it easy, and not to do anything strenuous so that they won’t harm their baby. On the other hand, historically speaking, most women worked hard till the last minute of their pregnancy until quite recently (and still do in some developing countries). We also know that many women gain too much weight during their pregnancy, and they have a hard time losing weight after childbirth. However, if they kept up their workout, they likely didn’t gain much, and found it easier to lose the extra pounds they gained during pregnancy.
Now, there is a green light to keep working out during pregnancy. One study conducted at the Polytechnic University of Madrid found that babies born to women who kept working out during pregnancy didn’t show any physical problems, and the mothers also stayed healthy. On the other hand, if a woman is sedentary and doesn’t do any workout during her pregnancy, the weight of the baby is positively correlated to the woman’s pre- pregnancy weight. It is not good for the baby; it is known that over-weight baby has a higher rate of being obese when she/he reaches maturity. This weight correlation between a mother and a baby was not found for the women who kept up their workout.
This means, if you are an over-weight mom, you really need to work out for your baby. You don’t need to do a hard workout. The participants in the study did only a moderate workout, such as a few times a week for 30-60 minutes. You can do that, especially if you consider your baby’s future.
By the way, if you are working out hard, you can keep the level reasonably high. Another study said that the level of workout during the pregnancy should be slightly less than before the pregnancy. This means that if you are running 20 miles a week for a long time, you can still run that much until you find it uncomfortable to do so, although your pace will probably become slower and slower as you gradually gain weight.
In any case, it is a good idea to check in with your doctor regularly during the pregnancy.
Source: Entrenamiento de fuerza muscular durante el embarazo y tamaño corporal del feto: un estudio aleatorio controlado. International Journal of Obesity, 33(9):1048-57, September 2009