I didn't ski while pregnant, but only
because I was living in Wash DC at the time, and I wasn't pregnant during much of ski season anyway (Dec and Jan babies).
I was coaching soccer then, though, and when I asked my doctor about the safety of doing so (eg, taking a ball or an elbow in the gut), he said that the baby is very well protected, and only something akin to the force of a car accident would disturb it. (Of course, such a force could
happen if you're hit by an idiot on a ski slope, but it isn't likely.)
More important, as others have said, is how your wife is feeling herSELF, as regards balance and conditioning and energy. She'll know when it's time to quit.
Everyone is different. I had to quit tennis at only 3 months pregnant, not because of physical issues, but because my timing was a little off, and then the hormones made me really really mad when I would miss easy shots. Physically I was healthy; mentally I was a basket case. After chucking a racquet or two into the fence, I knew it was time to take a break.
...On the other hand, I have many friends who played well into the third trimester. One of my best friends has big scabs on her knees in her "birthing video" from losing her balance and falling on the tennis court while 9 months pregnant. (Yes, she landed on her stomach, too, and went into labor the next day. Hmmmm.)