Many women continue to run during pregnancy and, unless they have been told by their doctor to stop due to a specific pregnancy-related condition, there is no reason why you can’t, as far as your baby is concerned.
However, pregnancy is not the time to begin running. Running is a cardiovascular sport that requires training to master. Women who continue running during pregnancy have the lung capacity and oxygen uptake to do what their bodies are already accustomed to doing.
That said, running is a sport that carries the risk of injury for everyone – and those risks increase for expectant moms. Once you become pregnant, a hormone called relaxin is released that makes all of your joints hyper mobile. Its job is to help your body expand to accommodate your growing baby and allow her to pass through your pelvis during delivery. While you’re pregnant, the relaxin in your system may make you more prone to strains and turned ankles when you exercise; moreover, running on uneven terrain can increase instability in your hips, knees and ankles.
There is also a lot of pounding that happens when you run, especially on cement sidewalks. The extra weight of your pregnant uterus puts a lot of pressure on not only your joints, but your pelvic floor as well.
Listen to your body, and consider scaling back to run/walk intervals, or walking uphill or on an inclined treadmill once you reach the fourth or fifth months of your pregnancy. As always, talk to your trainer or doctor about your particular situation.