I'd say it's a mix of being too cold/bad weather, ice jamming the waterways and ports through the winter to early May, less daylight, and a bunch of shops closed for the season. (I know one of our tour guides pointed out the 1 place that would be open in the winter, the rest are only open for the tourist season.)
Depending on how pregnant, she may not even be allowed to board. Definitely find out the rules for the lines you're looking at, and plan your trip or your procreating appropriately.
A good place to compare lines is Cruisecritic. The site is undergoing a refurbishment at the moment, though, which was supposed to be finished yesterday but seems to be behind schedule. Keep checking there. LOADS of BTDT info on all the different cruise lines, and about Alaska.
Besides the weather - ships are sailing elsewhere in the spring and fall.
In the 'summer', the dog sledding is done on the glaciers, which can only be reached by helicopter/plane--and can have a lot of cancellations if there is fog. You can do dog-cart rides without a flight. The dog team pulls a wheeled cart (ours seated up to 6 people in 3 rows) on a dirt track. Still not sure if it's something that's safe while pregnant...it's a pretty bumpy ride with dirt/pebbles coming at you, lol. We loved our ride, but yeah, our kids who were in the front row of the cart got the pebbles coming at them. We did the dog-cart ride up at Caribou Crossing Trading Post in the Yukon--it was part of our Chilkoot Charters excursion from Skagway--Bus up to the Yukon with various picture stops, short shopping stop, then a longer stop for lunch/dog cart/museum, etc, then the White Pass Scenic Railway back to Skagway. It was a fantastic excursion!
The excursion I mentioned in Skagway through Chilkoot Charters can be done without the dogcart ride--it's an 'add on'. The rest of that excursion is fine. Caribou Crossing Trading Post has puppies to hold and petting zoo type animals, as well as taxidermy animals in the 'wildlife' museum (we didn't have time for that due to the dog cart part.) Skagway also has the Klondike Gold Rush National park--free walking tours, etc. The White Pass Scenic railway is there--we really did prefer having the bus/train combo as I really don't need to be on a train that long!
We did great excursions, they weren't crazy expensive, and I think they would all be doable by a pregnant woman. Here is a link to a post in which I described our excursions on our Alaska cruise this June:
