I <3 my allotment!
Honestly, the hard work is in the beginning of the season - Spring! - in the digging and planting.
Once everything is planted, it's just maintaining - weeding and watering and enjoying the fruits of your labours.
Come autumn though, you should dig over and put in composts and other rotting matter ready for spring (though you still end up digging over in the spring anyways)
What kind of plot are you taking over? Is it pretty ready to go or is it needing a fair bit of work? I know in our allotment, the plots that become available are usually overgrown and untidy, as folks have just given up. I started my allotment from scratch and I reckon it took about 50 hours of work to get it into workable condition. (mine is small, only about 90 square metres). Each spring it probably takes about 30-40 hours of work to get it into workable condition (digging over beds, turning compost piles, digging in organic composts that have overwintered, etc) -this is before planting! Planting adds in another 20 or so hours, then in the summer, I spend about 5-10 hours/week weeding/watering. Little and often is better than big chunks of work at a time. I am a hugely busy individual and I can maintain my plot OK.
The real question is, will you be in physical condition to dig over the plot in March/April or will your hubby be willing to do so? Will you be wanting to to move your body so close post-pregnancy? What is your husband willing to do baby-wise to let you work on the plot? How close is the plot to your house? WIll you be able to get there easily? Then there is no prediction on how your baby will be. Will baby sleep easily and allow you to actually work on it or will baby be clingy-colicy-never sleep? Do you have any friends who may be willing to share the load with you? (check your allotment rules on this as they vary)
That said, raised beds will make life easier. Also, if the plot is yours (depends on the rules your allotment has for keeping it tidy -you will still probably have to cut down the weeds) - you can work it as much as or as little as you want and therefore, I would be tempted to take it, even if it means you just do a tiny wee bit this year to keep it- and then next year and subsequent years take on more and more.
Exciting!! There is NOTHING better than growing your own veg!!