I run very hard everyday but now I have found out that I am 5 to 6 weeks pregnant can I still run as hard? Will it cause me to miscarriage?

Since it sounds as though you are already in good condition, continuing with your running shouldn't be a problem.

You will likely find, however, that as time goes on you will need to modify your pace/distance. Your joints will become looser, and this can result in a greater chance of injury. You will also find a change in balance as time goes on and you will probably tire more easily. It is suggested, as well, to be aware of not elevating your body temp or heart rate too high while pregnant. The rule of thumb is to listen to your body and not push it as you might do when not pregnant.

It would be wise to speak to your doctor or midwife about it as well, since they would know any medical history and will be able to keep tabs on how your pregnancy is progressing and suggest any modifications they feel would be appropriate at different stages.

You might also want to check with your local hospital, birthing center, and Y about exercise programs specifically for during pregnancy.

You are wise to continue with some exercise as you will likely have an easier time with birth. Not necessarily faster, but you will have more endurance.