I jog about 3 miles several times a week and have for several years. Now that I am pregnant, will running cause me to go into premature labor?
I am glad to hear that you are in such good shape. What a great way to start pregnancy! In general, exercise is terrific for pregnancy. It helps with cardiovascular fitness, fights stress, and maintains your stamina as you get bigger. The more research that is done on exercise in pregnancy, the more we learn its benefits. In healthy low-risk moms, running doesn't increase the chance of preterm labor or any other serious pregnancy complication.
Many moms-to-be who, like you, are in good shape before pregnancy, can continue their usual routines for most of the gestation. That said, the principles of exercise in pregnancy are: stay well hydrated, don't overheat, don't do anything that risks falling or banging your abdomen, and don't work so hard you can't talk. So you will need to monitor your progress over time. You may find that you slow down a bit as you get bigger, but plenty of moms run right up until the end.
Your doctor or midwife can tell you if there are any specific reasons you should not jog during pregnancy, but in general healthy moms-to-be can do any activities that meet those four principles, including running. Enjoy!