Just about a century ago, pregnant women (well, pregnant women of a certain class) were cautioned not to exert themselves during pregnancy. Doctors were concerned that work or strain of any sort would stress the fetus. Women were cautioned not to write, not even to think, because using her cognitive powers would divert the woman's reproductive energy away from the babe growing inside of her.
Although our culture no longer has precisely this worry (and most women, including professors, are expected to perform their jobs up until they give birth), there is a residual worry that a pregnant woman's body is not able to sustain physical effort.
The article reports that Radcliffe continued to run while she was pregnant, but only under the close and constant supervision of a doctor. Nonetheless
"People were looking at her as if she were crazy."
Most pregnant women are cautioned by their doctors not to exercise strenuously or, sometimes, moderately or even at all. My OB told me to stop running and to walk instead. I continued to run through most of my pregnancy, as did several other women that I know. One ran until the day before she delivered her baby, and she ran a 10K when he was a couple of months old.
Radcliffe's doctor "allowed" her to keep running but told her to keep her heart rate below a benchmark and had her get extra ultrasound exams. Interestingly, the article also notes
Heart-rate precautions do not have scientific backing.
There is no scientific evidence to speak of concerning pregnancy and exercise. There are no controlled studies and very few epidemiological studies. In the absence of evidence, many obstetricians give out the same advice they were handing to women in the 1800s. At the same time, medical advice concerning exercise for other patients, who often have a real health problem, has changed across the board. Back-ache? Exercise. Arthritis? Exercise. Diabetes? Exercise. Elderly? Exercise. Depression? Exercise.
Given the anecdotal evidence and the new medical context which supports the benefits of exercise (for those who enjoy it, especially), it's hard to see any explanation for the cautions routinely handed out to pregnant women other than paternalism.