If your wife is active already then there should not be any problems up until the second trimester. Most women when they inquire with their doctors about activities are told that they can continue to do what they normally do as long as they are comfortable. i.e. if you are used to running you could continue (they would naturally slow down as it would become uncomfortable as time passed). I agree with the above posts that you probably would not want to go out in a blow.
JMO
Clayton
3 children and still sailing
P.S. I'm not a doctor and don't play one on TV. The above statements have not been qualified nor certified, quantities may vary from one location to another, taxes not included, no warranties included or implied.

Have Fun and good luck!