Why can't your wife ride at 18 weeks in? My wife rode to work the day before our son was born (she started to have some pre-labor pains and sent me home to get the truck to pick her up). At 18 weeks our son "rode" in the Cinderella metric century. Our OB/GYN and midwife both encouraged her to ride as much as she wanted. We had seen Sheila Young-Ochowicz compete at the 1984 Olympic trials while noticeably pregnant. I'm glad the women didn't crash as much as the men at that event.

I agree with the folks warning you off of the bike carriers and putting the baby into a trailer too early. I did a lot of walking during the first year and a half of my son's life, then we got a rear-steer tandem and got rolling. There is nothing more entertaining than the looks on people's faces when a two-year-old is the captain of a tandem and he takes his hands off the bars and crosses them on his chest.

I hope you are prepared for the sleep deprivation. It is well worth it, but that doesn't make it easy. I hope the pregnancy/birth experience works out well for all.