Been there, Done that ... TWICE!! and I survived and we had wonderful trips each time.
Since 2003 we have taken the following trips to WDW:
Went when I was 6 months pregnant with daughter.
Went next when she was 16 months old and I was 4 months pregnant with my 1st son.
Went next with a 2 year old and a 9 month old.
Went last time with our 4 year old daughter, our 2 year old son and 7 months pregnant with 2nd son.
In April we are heading down with our 5 year old, our 3 year old and our 1 year old and I can't wait!!
Go and have a FABULOUS trip!!
Advice would be:
1. Start the days early and leave the parks around lunch time with a nice long break in the room or by the pool for the heat of the day. If everyone is up to it, head back to the parks for the evening.
2. Make sure your wife stays hydrated and takes plenty of opportunities to sit and get off her feet.
3. Take advantage of the Baby Centers at each park -- wonderfl places to cool of and take a break with lots of things to help out with your son -- changing tables, microwave, high chairs, tv, etc.
4. Suprise your wife with a trip to one of the spas for a massage or pedicure if she likes them. I love that at any time, but while pregnant they are even better!
Hope you have a wonderful time!