The thermometer reads -15C, but the biting wind makes it feel more like -30C.

The sidewalks are covered with a thick sheet of ice and snow.

These are no conditions for running, but more than a few brave souls lace up their running shoes on these frigid days to tough it out on wintry streets.

The same used to be said of pregnancy – it was no condition for running. But more and more women – with the blessing of their doctors – are sticking with their favourite sport during pregnancy. They aren’t training for marathons any more, but they are running to stay active and relieve stress during their pregnancy.

“It’s one thing to start up a walking program during pregnancy and another thing to start running for the first time,” Jim Pivarnik, director of the Centre for Physical Activity and Health at Michigan State University who has studied exercise in pregnant women, told the U.S. News and World Report. “But if you’re having a healthy pregnancy and are already a running, there’s no reason to go from 60 miles a week to running.”

The Society of Obstetricians and Gynecologists of Canada says most women runners can continue to run while pregnant without doing any harm to the growing baby. However, if pregnant women feel any pain, particularly in their pubic region, during a run, they need to stop because it’s a sign their changing bodies aren’t adapting well to the exercise.

There are other medical conditions which should cause pregnant women to pause before doing any exercise, including running. Women with heart problems, high blood pressure, problems with bleeding or controlling blood sugar during the pregnancy as well as those carrying more than one baby or at high risk for preterm labour should check with their doctors before heading out for a run or any other kind of exercise.

Once pregnant women get the okay from their doctors to continue running, there are a few other modifications they need to make to their workout routine.

Joan Marie Butler, author of Fit and Pregnant: A Pregnant Woman’s Guide to Exercise, said expectant runners need to take a few extra minutes to stretch before and after their workouts to prevent injuries. The hormone relaxin, which causes joints and ligaments to loosen during pregnancy, can also make them vulnerable to injury, so a little stretch can go a long way.

Butler, who is also a nurse-midwife and master’s athlete in Syracuse, N.Y., also urged women to back off the speed, intensity and frequency of their runs, especially as the pregnancy progresses.

“Remember, you are running to maintain your fitness, not to train. Slow down – don't push your pace, and don't push your distance. Back off running a preset course if you just don't feel like doing it,” Butler wrote.

British marathoner Paula Radcliffe, who won the New York marathon an astonishing 10 months after giving birth to her daughter, ran up until the day before she gave birth in 2007. But the elite athlete was careful to slow the pace of her runs during her pregnancy and she gave up one of her twice-daily runs.

“Because I’m an athlete, I’m quite in tune with my body, and I feel in tune with the baby,” Radcliffe told the Times Online after running a 10K charity race while she was six months pregnant. “The advice I’ve got is that it’s good for me to stay fit in pregnancy by running and it’s good for the baby as well.”

Butler, author of Fit and Pregnant, advised women to stop running if they feel Braxton Hicks contractions or ligament pain. “At seven months, I sometimes felt Braxton Hicks contractions during the first few minutes of a run. I would stop and walk a few minutes and then, when the contractions stopped, start up slowly again,” she wrote.

While many women battle nausea and fatigue, especially early in their pregnancy, Butler said some of the runners she interviewed for her book actually felt better after a run. But if your body is telling you to nap, listen to it or try a gentler exercise, such as walking or riding a stationary bike.

Pregnant women will also need to invest in a supportive sports bra to accommodate sore, swollen breasts, Butler advised. They may also want to tailor their routes around bathroom stops. Growing babies are known to send their mothers on frequent bathroom breaks.