Recommendations for exercise during pregnancy are vague, and generally quite conservative so as to prevent any undue stress on the pregnancy or the expectant mom. The official medical position on the subject has been revised in recent years, however, to encourage more activity during pregnancy. The American College of Obstetricians and Gynecologists (ACOG) recommends that pregnant women get in at least 150 minutes of moderate-intensity aerobic activity every week, which you could break down into 30-minute workouts five days a week, or shorter 10-minute sessions throughout each day. "Moderate," as the ACOG defines it, means you're moving enough to raise your heart rate and start sweating, although you can still talk normally (but you shouldn't be able to sing).
The general "rule," if there is such a thing, is that women shouldn't begin a new activity during pregnancy, but that it’s safe to continue most activities while pregnant as long as the body was accustomed to that activity on a regular basis pre-pregnancy. Some exceptions to that are high-impact activities that have a high risk of fall or injury, like horseback riding, downhill skiing or contact sports like soccer.
This “rule” can be applied to most moms in most situations—but there are the outliers: those who do everything a little more to the extreme. What about the marathon runners, the power yogis and the competitive cyclists? For many women who are accustomed to advanced physical exercise pre-pregnancy, there is little in the way of studies or official recommendations to guide them on how much is too much. In those cases, as in most, I defer to the sensibilities of the individual woman. One of my most frequent phrases is “listen to your body."
A marathon runner can usually keep training well into pregnancy, sometimes until the last days, but she also usually has to adapt her routine throughout the pregnancy to continue without discomfort or injury, like slowing the pace, cutting the distance, alternating walking and running, and adding an under-belly support to lighten the strain of the weight of the bump as it grows. When will she know to make these changes? When her body tells her so! It’s important to be attentive to the subtle signs: shortness of breath earlier in the run than usual, joint or pelvic pain and overall discomfort.
In other cases, it may not be so clear. The power yogi who is ultra flexible and accustomed to twisting into pretzel shapes, back bends and headstands can keep up her yoga practice throughout pregnancy, but with modifications so that she doesn’t over-stretch the ligaments that are already lax because of hormonal changes. While it’s important for these moms-to-be to listen to their bodies and not go so deep into Warrior 2 that they feel pelvic pain, it's also important that they know the risks of some positions or poses and take precautions to minimize risk of injury, such as using a prop for balance for all standing balance poses and avoiding extreme extension of the spine to protect the vulnerable lower back and prevent separation of the abdominal muscles.
Of course, you should always consult your doctor before starting any exercise routine during pregnancy. And when you do, be sure to let your doctor know what you were doing for exercise before pregnancy so that the recommendations will be more tailored to your situation. For those who are looking for a good exercise to get and stay in shape, my favorites that are safe for nearly any healthy mom-to-be are walking, swimming (or any kind of aquatic exercise), and prenatal core exercises to target the muscles weakened the most during pregnancy.