As readers of this blog know, fitness during pregnancy is one of my personal crusades. I ran 5 miles this morning at 24 weeks, and plan to continue that pace more or less as I enter my third trimester. I have found that exercising about an hour a day, vigorously, is key to keeping my weight gain on track.

I also know that this kind of exercise schedule is pretty rare among pregnant women. According to a new report from the Institute of Medicine, about two-thirds of women of childbearing age are overweight, with about half these women clocking in at obese. The IOM decided to set new guidelines recently to discourage excessive weight gain among these women. Overweight women, the new guidelines say, should only gain 15-25lbs, and obese women should gain 11-20.

These are tough limits to stick within unless you’re doing some serious exercising and watching what you eat. Of course, if you’re starting out overweight or obese, chances are you’re not doing that. And since many people seem to think pregnancy is an all-purpose excuse to be inactive and eat a lot, few pregnant women start up a vigorous exercise program. Consequently, only about a quarter of women stay within the recommended weight limits.

I think this is a shame. Excess pounds put on during pregnancy can be very hard to take off, and even small amounts of excess weight cause lots of health problems. Personally, I think that inactivity during pregnancy should be viewed the same way that smoking during pregnancy is. I suspect the overall medical outcomes aren’t orders of magnitude different.