I wanted to quickly chime in on this one. I feel like there is so much pressure on women these days to continue to train during pregnancy and the bottom line is you have to do what is right for YOU. I got pregnant only a few weeks after an ironman so needless to say I was in great shape. After 3 months or so, I had to cut running out completely due to ligament pain. You know what? That's OK. Staying active while pregnant is important, setting goals for duration or distance is not. It's OK to slow down and focus on growing a human. Good luck with everything!!
It totally depends on the person and even the pregnancy. I've had two babies and my experiences were very different - and both were great. With my first pregnancy, I had almost completely stopped running in order to get pregnant due to fertility treatments, so when I finally got pregnant, I didn't want to start running again. Instead, I swam 3x per week, snowshoed, did prenatal exercise videos (light cardio and weights) and walked all the way through my pregnancy - felt great, gained 20 lbs, and was back running 6 weeks post c-section (due to a breech baby - not related to running or not running!). My return to running was pretty smooth - I ran an easy paced half at 5 months post-partum about 20 minutes slower than my PB at the time and a PB marathon when my daughter was 13 months old.
The second time around I had been training right up until pregnancy, so I just kept running. But, I wasn't training. I ran for fun and fitness and because it felt good - my "long" runs were 10-15 km until about the half way mark, then all my runs were shorter, easy efforts. I kept going right to the end - by then, maxing out at about 2-3 miles at a time, about 60-90 seconds slower than my old "easy pace". I listened to my body and adjusted accordingly.
This time, I was back running after a month, ran that same half about 15 minutes faster than last time at 4 months post partum, and set a 14 minute PB in the marathon when my son was a year. My return was quicker partly I think because I'd run through.
On the flip side, I've had several friends who are excellent runners who were super fit going into pregnancy and could hardly run a step due to various reasons. Pregnancy can do a number on your body - there's no reason not to stay active, but if running doesn't work for your wife, there are lots of other options! Good luck!