How much is too much exercise when you’re pregnant?

The caption on Lea-Ann Ellison’s photo says it all: “8 months pregnant with baby number 3.”

“I have been CrossFitting for 2½ years,” Ellison posted on CrossFit’s Facebook page, “and … strongly believe that pregnancy is not an illness, but a time to relish in your body’s capabilities to kick ass.”

The photo of the 35-year-old former bodybuilder from California prompted a slew of comments – both positive and negative.

“This is shocking and not in a good way. Lifting heavy things during pregnancy is dangerous to you and your baby,” Natalie Rose wrote.

“Why would you risk hurting your baby just to stay in shape?” Stephanie Herrera asked. “This is the stupidest thing I’ve ever heard.”

Others jumped in to defend the fit mom.

“I’m 6 months pregnant with triplets and am still Crossfitting as much as I can!” Carol Bolliger shared.

“Doctors say it is perfectly fine to stick to your exercise route while pregnant, in fact it is encouraged,” Kristen Funk wrote.

Exercise is encouraged during pregnancy, says Dr. Siobhan Dolan, an ob-gyn and medical adviser for March of Dimes. Ellison’s routine is an extreme example, but most moms can benefit from aerobic activity and strength training before and after childbirth, she says.

“A woman’s overall health, including obstetric and medical risks, should be evaluated before prescribing an exercise program,” the American College of Obstetricians and Gynecologists’ guidelines say. “Generally, participation in a wide range of recreational activities appears to be safe during pregnancy; however, each sport should be reviewed individually for its potential risk.”

In general doctors recommend 30 minutes of moderate physical activity most days of the week, Dolan says. What “moderate” means varies from person to person, and depends on how active someone was before becoming pregnant. Competitive athletes, the gynecologists’ organizationnotes, may be able to perform at higher rates during pregnancy and return to vigorous activity sooner after giving birth.

It would be great if everyone got in shape and started exercising before becoming pregnant, Dolan says. “But in the real world, I get it, you’re working, you’re busy.

“Pregnancy is a great motivator,” she says, but women who have never exercised before should be careful about starting a strenuous program right off the bat. Walking is a good way to start; you can begin slowly and then build up.

There are certain things pregnant women should avoid while exercising. Activities with a high risk of falling or abdominal injury, such as horseback riding or downhill skiing should be avoided, as should scuba diving.

Lying flat on your back or on your stomach can slow blood flow back to the heart, Dolan says, so pregnant women should also modify these exercise positions.

Modification can help you keep up with your normal workout routine. For instance, sit-ups can be done on the side instead of on the back. There’s even a website dedicated to WODs – or workouts of the day – created specifically for CrossFit moms.

“CrossFit is a strength and intensity-based fitness program,” a warning on the site says. “However, during pregnancy you want to concentrate on strength and keeping your body healthy, rather than the intensity.”

Intensity is the bigger concern about what Ellison is doing, says Dr. Raul Artal, chairman of St. Louis University School of Medicine’s Department of Obstetrics, Gynecology and Women’s Health as well as the lead author of the American College of Obstetricians and Gynecologists’guidelines for exercise and pregnancy. Lifting big weights – in excess of 15 pounds – could put both mom and baby at risk, he says.

When lifting weights, Artal explains, you divert blood flow from internal organs, including the uterus, to your muscles. That can prevent oxygen from getting to the baby. He compares it to stepping on the umbilical cord for 20 or 30 seconds, or however long you are exerting yourself.

Weightlifting can also put the mom at risk for premature labor, Artal says. Bearing down could potentially lead to uterine activity – i.e. start contractions – or rupture membranes in the gestational sac, which surrounds the embryo in early pregnancy.

“What’s important to point out is that individuals may get away with this activity and nothing will happen,” he says. “What’s difficult for doctors to predict is which mother will have a problem.”

While Artal says he would advise women not to engage in this type of activity any time during the pregnancy, “this is a very personal decision. A woman would have to decide if she’s willing to take the risk.”

Ellison has obviously made her choice. “Haters will hate and it’s ok. My life is not their life thank goodness,” she posted on Facebook.

CNN’s Miriam Falco and Anna Gonzalez contributed to this report.