Exercise and Miscarriage
You should always talk with your doctor about the appropriate level of exercise if you are pregnant. Some exercise is good during pregnancy as long as your cervix hasn’t been found to be weak. Most of the research I’ve read recommends moderate exercise. Try to avoid anything that is going to raise your core temperature or anything that might involve falling or other trauma (such as skiing). I did very little exercise when I was pregnant mainly out of fear and first trimester nausea. However, I probably could have benefited from getting at least some exercise.
There does seem to be some conflicting information about whether or not exercise is associated with miscarriage. However, according to one study, women who engage in high impact exercise in early pregnancy (more than 7 hours per week) were three times more likely to miscarry. I actually had a PA in my OB’s office say that I could run a marathon after becoming pregnant and it wouldn’t make a difference. I don’t believe that and I think women should be cautioned to take it easy. It just makes sense that it would be harder for an embryo to implant and take hold properly if your body is bouncing around.
 Madsen M, Jørgensen T, Jensen M, Juhl M, Olsen J, Andersen P, Nybo Andersen A. Leisure time physical exercise during pregnancy and the risk of miscarriage: a study within the Danish National Birth Cohort. BJOG 2007;114:1419–1426