While you shouldn't race during pregnancy, there are many women who do so early in pregnancy. There are also many case studies of elite athletes tolerating high levels of endurance and interval training without compromising fetal growth or complicating their pregnancy.
Second Trimester
During the second trimester, the size of your uterus increases rapidly to accommodate the growing fetus. Accompanying this rapid growth, pregnancy can become more uncomfortable as your body makes room for the growing uterus and the muscles and supporting pelvic structures are stretched. By the end of the second trimester, the fetus grows to almost four times the size it was at the end of the first trimester. Maternal and fetal oxygen and nutrient needs increase during this time of fetal, uterine and placental growth.
Cardiac output continues to rise during the second trimester. By 20 to 24 weeks of pregnancy, cardiac output is 30 to 50 percent greater than before pregnancy. By the middle of your second trimester, your stroke volume peaks by as much as 30 percent compared to pre-pregnancy levels. Total blood volume continues to rise. These two adaptations allow for adequate nutrients and oxygen to be supplied to the fetus both at rest and when running.
During pregnancy, blood flow is distributed preferentially to the uterus, kidney and skin and increases with gestational age. Running results in a redistribution of this blood flow away from the uterus to skin and exercising muscles and is directly proportional to the workout intensity.
These opposing effects could compromise blood flow to the fetus and affect fetal growth and development. However, thanks to the increases in cardiac output and blood volume and placental adaptations that result in greater extraction of oxygen and nutrients, these effects are minimized, particularly with running at moderate intensity. Studies have shown that placental adaptations are greater in women who maintain a regular exercise program through the second trimester of pregnancy.
Choose running routes and surfaces that are flat with few obstacles to minimize the risk of falling. In most cases, you can keep your training the same in the second trimester as in the first, and you can even increase your total weekly running mileage by 5 to 10 perent if you tolerate the mileage without increased fatigue, shortness of breath or back or joint pain.
As your uterus gets larger, your center of gravity shifts forward, making you more prone to lose your balance. If you have a history or are at risk of preterm labor, proceed cautiously with strenuous activity in the second trimester.