I'm another one still doing squats, with the same weights as prepregnancy so far, at 25 weeks. As Stebby mentioned, there is not a muscle or bodily organ called the valsalva. What your OB was probably referring to was the "Valsalva maneuver," which means holding your breath during the exertion phase of an exercise (if you watch the way some of the Olympic power lifters lift weights, you'll probably see this in action). This increase intra-abdominal pressure, which obviously no one with a baby growing inside of them needs. As long as you breathe out during the exertion phase of the movement (i.e., as you stand from a squatting position), you shouldn't have any problem
However, squatting with weights can put extra pressure on your PC muscles (the ones you do Kegels for) -- I mean, more pressure than they are already under just from the growth of the uterus itself. I believe Sheila has said on this board in the past that as long as you can do a Kegel contraction during the squat, you're OK -- if not, you should find an alternative exercise.
Hope that helps!