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Extreme fitness during pregnancy: How much exercise is safe?

Instagram star and mom-to-be Hannah Polites is drawing much attention -- and some criticism -- over her super-fit pregnancy selfies.

Polities, a self-described "fitness junkie" and lifestyle blogger, has shared several photos of her growing baby bump since becoming pregnant about six months ago with her 1.2 million Instagram followers.

But her insanely buff body -- and relatively small stomach as she approached her third trimester -- have garnered some negative comments, accusing her of putting her unborn child at risk.

Yet, Polites insists she -- and her baby -- are healthy. "I'm by no means trying to put on muscle but would like to maintain fitness where possible to help me be as strong as possible for my birth, baby and for myself!" she stated in an Instagram post. "I'm mainly sticking to Pilates, strength training and yoga but every pregnancy and body is different."

In another post she wrote: "For everyone saying my baby is small and malnourished scans are showing he/she is actually 20% bigger than the average at this gestation."

Still, the uproar over her photos does beg the question: how much exercise is safe during pregnancy? The answer, it turns out, depends on the woman.

"It all depends on the baseline they're starting at," Dr. Aaron Baggish, director of the Cardiovascular Performance Program at the Massachusetts General Hospital Heart Center, told CBS News. "Pregnancy is not a great time to be working on improving athletic performance, but it's an awesome time to be thinking about how to maintain it."

Baggish regularly counsels women who are competitive or recreational athletes before and after they become pregnant.

He said he would never recommend a woman intensify an exercise regimen or shoot for new goals during pregnancy. "But if they come in with a very accomplished base and they know their body and they're willing to listen to their body, then most women can continue to be very, very active all the way up until the end of their pregnancy," he said.

Still, there are certain types of exercise pregnant women should avoid. The American College of Obstetricians and Gynecologists recommends steering clear of contact sports and activities with a high risk of falling like downhill snow skiing, water skiing, surfing, off-road cycling, gymnastics, and horseback riding. And it warns pregnant women against scuba diving and "hot yoga" or "hot Pilates."

Baggish also recommends avoiding weight lifting and high-intensity interval training from the middle of the second trimester and beyond.

Yet most forms of exercise, he said, can be tailored to meet a pregnant woman's needs.

"Yoga is a classic example," Baggish said, "There are a number of yoga poses to avoid, particularly in the late second and third trimester, which involve directly laying on the back or stomach, but yoga can certainly be modified. Good yoga instructors will work with pregnant women and can put them through rigorous yoga sessions that are still satisfying and fun without doing any harm."

But if women notice they are starting to feel sluggish after exercising, that's a signal that it may be time to slow down.

"One of the first things to remember is that exercise done at moderate, sustainable intensity duration leaves you feeling energized for the rest of the day, not flat out tired," Baggish said. "So if a woman is left dogged and drained for the day, that's an indication she should back off."

Women should stop exercising and call their obstetrician if they experience any of the following symptoms: bleeding from the vagina, feeling dizzy or faint, shortness of breath before starting exercising, chest pain, headache, muscle weakness, or calf pain or swelling.

Finally, Baggish emphasized that exercise during pregnancy has numerous health advantages, and far from being afraid of the risks, women who do not regularly work out could benefit from doing so before and during pregnancy.

"Women who exercise tend to have healthier babies. They have smoother deliveries and are more likely to be able to deliver vaginally," he said. "If there are complications, they tend to have babies who recover more quickly. So there are a lot of reasons for both mom and baby why the right amount of exercise is an important part of a healthy pregnancy."

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