Regular exercise for up to seven or more hours per week is NOT associated with increased rates of miscarriage in the first or second trimester. Woman can do low or high-intensity exercise including aerobics, jogging, cycling and swimming without risk of miscarriage. Bending and lifting at work is also not associated with increased risk of first-trimester (the first 13 weeks of the pregnancy) miscarriage.
These were the findings of a large six-year study in Denmark with 90,000 women. No duration of exercise or gestational age was correlated with miscarriage.
The American College of Obstetricians and Gynecologists recommends that active women continue to exercise at moderate intensity for 30 minutes on most day of the week during pregnancy. Women who perform strenuous routines should be followed closely. Also women who did not exercise before pregnancy should be evaluated by their doctor before beginning an exercise routine.
Medical reasons that you should not exercise are significant heart disease, restrictive lung disease, incompetent cervix, multiple gestation at risk of premature labor, persistent vaginal bleeding, placenta previa after 26 weeks pregnant, premature labor during the current pregnancy, ruptured membranes and preeclampsia or pregnancy-induced hypertension.