Is It Safe to Exercise During Pregnancy?

An hour of cardio usually flies by for me at the gym, thanks to my secret motivational strategy: watching "Law & Order" reruns on the club's TV. I hop on the elliptical machine as the opening credits roll, and before I know it, Sam Waterston is finishing his closing argument to the jury. At least, that was the case before I got pregnant. In my first trimester, some days, to my amazement, I'd poop out 10 minutes into the show—before the detectives even identified the body.

"Many active women are surprised at how pregnancy affects their workouts," says Renee M. Jeffreys, M.Sc., a prenatal-fitness consultant in Milford, Conn., and co-author of Fit to Deliver (Hartley & Marks). "But remember that these are normal, short-term changes."

So should you dial down your cardio? Are certain machines off-limits? Can you still do Pilates? We’ve answered your most pressing questions.

Exercise During Pregnancy Benefits

Of course, exercise during pregnancy depends largely on what your fitness level is, which trimester you're in, and how you're feeling, Jeffreys says. But this much is certain: The gym is a great place to be when pregnant. If one cardio machine or strength exercise isn't comfortable, there's always another one to try.

Getting yourself to the gym may take an extra dose of motivation, but the payoff is huge. Consistent exercise during pregnancy can minimize aches and constipation, help you sleep better, and lower your risk of gestational diabetes and depression. You may even end up having a shorter, less complicated labor. Developing good workout habits during pregnancy will help you get your body back faster after delivery too.

What’s more, exercise keeps your endorphins (your body's natural "happy" chemicals) flowing, says prenatal trainer Erinn Mikeska, owner of Delivering Fitness, in Dallas. That's a crucial benefit, especially since we now know that there are more mood disturbances during pregnancy than postpartum, due to the massive influx of extra estrogen and progesterone (hormones linked closely to mood), says Melanie Poudevigne, PhD, health and fitness management program coordinator at Clayton State University, in Morrow, Georgia.

Will Exercise Harm My Baby?

Many newly pregnant exercisers worry about miscarriage, thanks to age-old myths that say a bout of strenuous exercise can harm the baby. But "there is no real evidence that exercise is linked to miscarriage," says Bruce K. Young, MD, coauthor of Miscarriage, Medicine & Miracles (Bantam) and professor of obstetrics and gynecology at New York University School of Medicine. However, early in pregnancy, elevating your core body temperature may be damaging to the fetus, so stay hydrated, don't exercise outdoors in the heat of the day, and avoid huffing and puffing so hard that you can't talk.

Heavy exercise isn't going to hurt your baby, but it will tire you more quickly than it did pre-pregnancy. Blood volume doubles during pregnancy, and a woman's heart needs to work harder to push all that blood around—including circulating it through the placenta, an extra organ. "That means the stress on your heart will be 50 percent greater for the same exercise that you were doing before pregnancy," Dr. Young says.

How Will I Feel?

Pregnant women often notice that they feel out of breath more quickly than they used to. You may assume this is a sign that you're out of shape. In fact, during pregnancy, you're breathing 20 to 25 percent more air because you need to get rid of the carbon dioxide levels in your own blood – and in your baby's. (Babies in utero aren't breathing on their own, but they're still producing carbon dioxide, which transfers to the mother's blood.) "So breathing more doesn't mean you're any less fit," explains Dennis Jensen, PhD, lead researcher on a Queen's University study of exercise and respiratory discomfort during pregnancy. It simply means that your body is adapting exactly as it should.

You may also feel less balanced when exercising while pregnant. "Looser joints and a changing center of gravity as the pregnancy progresses alters a woman's sense of balance, making her more susceptible to injuries such as sprained ankles and knees," says Shannon Clark, MD, associate professor in the department of obstetrics and gynecology at the University of Texas Medical Branch (UTMB) in Galveston. For any high intensity workouts, find an instructor knowledgeable on how to alter workouts to accommodate pregnant women.

Pregnancy isn't the time to push yourself to the max, but it's also okay—and good for you—to get your heart rate up with cardiovascular exercise. Although a target heart rate of 140 is a number that's often cited, there's no precise number to shoot for.

Best Exercises During Pregnancy

If you have access to prenatal exercise classes, sign up. Not only are the workouts modified for pregnancy, but you also get to bond with your fellow moms-to-be over charming symptoms such as heartburn, swollen feet, and hemorrhoids. You might even get labor tips.

If your favorite classes don't come in the prenatal variety, it's fine to keep going, as long as you pay attention to how your body feels, limit your intensity, and stay within the normal range of motion. Just make sure the instructor knows you're pregnant and is knowledgeable about modifications you can make, Jeffreys advises. Also, if your instructor hasn't worked with pregnant women, find one who has.

Do you want to start working out while pregnant? Here are the most common classes you'll find at the gym:

Pilates

Pilates helps maintain your abdominal muscle tone, which will support your growing belly, minimize back pain, and give you more oomph for pushing during labor. But mat classes can be problematic after the first trimester because so much work is done lying on your back. Either opt out of these exercises or use an angled foam spine support (found in most Pilates studios but not many gyms); this will keep your head higher than your belly. You can still do the side-lying leg work, upper-body exercises, and stretches.

Yoga

Prenatal yoga not only strengthens your core and improves flexibility, but with its gentle movements and emphasis on breathing and meditation, it also fosters a sense of calm. In the second half of your pregnancy, avoid exaggerated twists and movements that tug on your belly, moves that require you to lie on your back or belly for prolonged periods, and inversions like headstands and shoulder stands.

Water Aerobics

You can't trip and fall; you won't overheat; and for once you won't feel like a big clod. No wonder water aerobics is a third trimester favorite. Your joints will thank you! Wear aqua shoes so you don't slip on the bottom of the pool.

Strength Exercises

Worried that resistance (strength) exercises will cause joint injury? It's true that pregnancy floods your system with relaxin, a hormone that loosens ligaments to prepare your body for delivery. But a 2011 University of Georgia study found that a low-to-moderate-intensity strength program is safe, even for novices. "The relaxin risk is largely theoretical," says study co-author Patrick O'Connor, Ph.D. Though the study mostly involved machines, free weights and body-weight exercises such as push-ups and squats are safe, too. (No, squatting won't trigger labor.) O'Connor notes that the women were supervised and recommends that anyone who is new to weight training should be as well or consult videos to learn proper technique.

If you want to train your core, know that doing crunches (or other exercises) on your back is a no-no after the first trimester: Your growing uterus can compress the vena cava, the major vessel that returns blood to your heart, potentially reducing blood flow and making you feel dizzy or nauseated. O'Connor's study included an ab exercise performed while standing: The women exhaled and then drew in their navel toward their spine "as if they were trying to button up pants that were too tight in the waist," he says.

Cardio

When it comes to cardio exercise, Fit to Deliver co-authors Karen Nordahl, M.D., an Ob-Gyn in Vancouver, British Columbia, and Renee Jeffreys have a rule of thumb: "If you were really active before pregnancy, stay really active. If you weren't, now is a good time to become active." For beginners, Nordahl recommends 30 minutes of walking three days a week.

Here’s the lowdown on cardio machines for exercise during pregnancy:

Treadmill: Walking on the treadmill is ideal since you can control the terrain. Add moderate hills when you're up to it; go flat when you're not or if hills trigger calf cramps. If you're a runner, let your body tell you when it's time to switch to walking; nearly everyone does. And don't worry about "shaking your baby loose," since she's plenty safe swimming around in amniotic fluid while you jog at the park.

Elliptical: The elliptical trainer places little stress on your joints. However, the motion may feel uncomfortable if you're experiencing symphysis pubic dysfunction (SPD), a pain in the front of your crotch.

Stationary bike:The recumbent and upright bikes are both good options. Many women like the back support the recumbent offers, though in the third trimester your belly might get in the way of your knees.

During pregnancy you'll need to scale back on the intensity. Gauge your intensity using the Rate of Perceived Exertion (RPE) scale from 0 to 10: Aim for an intensity between 3 and 5 (you should be able to talk but not belt out show tunes).

Ignore heart-rate readouts on the cardio machines; since your blood volume increases during pregnancy, and your resting heart rate is considerably higher than normal, heart rate isn't an accurate gauge of intensity.

Exercises to Avoid While Pregnant

While there are some things to avoid, such as scuba diving, horseback riding, or any contact sport that could cause blunt-force trauma to the abdomen, there's relatively little that pregnant women can't do.

Even the longstanding prohibition against exercising on your back is somewhat of a myth, Dr. Young says. It's true that lying flat on your back late in your pregnancy can cause your growing uterus to push down on the veins whose job it is to deliver blood, leading to decreased blood flow.

"Blood can get shunted away from the uterus, and you might feel light-headed," Dr. Young says. But performing exercises on your back for a short period (such as a series of Pilates moves) is not likely to do any harm, and you would feel uncomfortable long before your blood flow was compromised, he explains.

Warning Signs: Working Out While Pregnant

Pregnant women who exercise should monitor signs of potential problems, keeping in mind that if there's something wrong with the pregnancy, it's not caused by exercise. Rather, exercise might provide that extra bit of stress that brings the problem to the forefront. Stop exercising and consult your doctor if you have:

• Vaginal bleeding
• Abdominal cramping
• Light-headedness
• Excessive nausea
• Extreme headaches

Also watch out for dehydration, which can cause preterm uterine contractions, and in the worst-case scenario, lead to preterm labor. Eight or nine cups of water a day is recommended during pregnancy, on average, but fluid needs should be adjusted for intensity and duration of the workout.

• RELATED: 14 Reasons to Call the Doctor during Pregnancy

You may also need to be careful—or avoid exercise altogether—if you have one of several pregnancy complications, including:

• Significant heart or lung disease
• An incompetent cervix or cerclage
• You are carrying more than one baby and are at risk for premature labor
• Persistent second- or third- trimester bleeding
• Placenta previa past 26 weeks of pregnancy
• Premature labor during your current pregnancy
• Ruptured membranes
• Preeclampsia (pregnancy-induced hypertension)

A Trimester-by-Trimester Guide to Exercise During Pregnancy

First Trimester Tips

• You can pretty much continue doing exactly what you were doing before pregnancy (including lifting the same amount of weight).
• Now is a smart time to get involved in a prenatal yoga or Pilates class; it's a great habit to carry throughout your pregnancy (plus, you'll get to be around other pregnant women).
• If you feel sluggish, try decreasing either the intensity or the duration of your workout rather than skipping the workout altogether (example: drop your jogging pace by 30 seconds a mile, or cut your bike ride a few miles short).

Second Trimester Tips

• Your heart is starting to work harder to circulate all the blood you built up during the first three months. Cut back your cardio intensity by 20 to 30 percent, says Dr. Young. You'll still feel as if you're working just as hard.
• Avoid inversions (such as Downward Dog in yoga) if you feel at all light-headed.
• If you're a cyclist, consider switching to the stationary bike or to a Spinning class; your growing belly can make balancing on a bicycle tricky.

Third Trimester Tips

• Your joints are more vulnerable, so beware of heavy (15 pounds or more) weights; opt instead for more repetitions.
• Do free-weight exercises seated, if possible, because you'll want your back supported (plus it's hard to balance while standing up).
• You can continue with your cardio right up until you deliver, but don't be surprised if you can walk faster than you can jog. Many pregnant women find that supporting their belly (with something like the Belly Band) during cardio helps take the pressure off.
• Now is a great time to try swimming: You'll feel wonderfully weightless in the water, and it won't stress your joints.