For most women, the reasons for not doing high impact activities are, like you said, more because the mother may either hurt herself or fall. In a normal pregnancy, the baby is pretty safe in his bubble of amniotic fluid, and if you are paying attentionto how your body feels while also heeding the ACOG guidelines for prenatal exercise, you are probably fine.
That being said, most women find high impact activities uncomfortable at some point... you've got a lot of extra weight pulling at muscles, joints, and ligaments that are already more lax due to the hormone relaxin... relaxin also makes you more prone to sprains.
Finally, all of the extra weight of your growing uterus, baby, placenta, amniotic fluid, etc., is bearing down on your pelvic floor, so if you have even the slightest bit of incontinence while doing impact activities, you'll want to take it down a notch until after you've had the baby AND have rehabilitated your pelvic floor (kegels).
Some women can continue doing high impact throughout their pregnancies with no problem, but most cannot. The good news for low impact activities is that you have a growing "weight plate"... will find that you'll still be able to get a decent workout doing low impact activities. There are tons of low impact activities you can do to get a good workout with risking your joints all of the extra weight combined with high impact. I've run into a couple of women who felt that they'd "done in" their knees by doing too much impact during their pregnancies.
A good plan of attack is to have a bunch of different workout activities in mind so that, if one becomes uncomfortable, you can go on to another. In my most recent pregnancy, for instance, I did light jogging through my first trimester (lots of water!!)... taught low impact prenatal/postpartum classes through almost all of my pregnancy... did resistance exercises throughout my entire pregnancy... did some step aerobics until 6-7 mos... did the elliptical trainer until about 8 mos... and swam laps through my entire 3rd trimester (until the last week or two when I quit altogether...). If I ever get pregnant again, I'll begin the swimming earlier... it was an awesome workout, and it felt great to get the weight off of my feet and back!
BTW, in teaching postpartum women, I've found that high impact jacks are the #1 move that causes women to experience urinary incontinence... this leads me to believe that this particular move puts more strain on the pelvic floor than most others.
Susan
Healthy Moms Certified Perinatal Fitness Instructor
Birgitta Gallo's Expecting Fitness is another excellent book. It actually has an entire chapter devoted to the various possible fitness activities, which should be avoided, and which should be modified ( and how ) during what part of your pregnancy. I would consider both to be absolutely essential reading for the fit preggo. 