I work in wellness professionally and am a yoga teacher so I wanted to add something to this thread.
First of all, there are lots of ways to get "exercise" that don't include straight up cardio or going to the gym. The basic guideline is to shoot for 30 minutes of moderate movement a day, and you can break this down into no lower than 10 minute increments.
So... getting up and really moving around the house in a whirlwind (or a pregnant lady's whirlwind to be exact) for ten minutes COUNTS!!! In my company we encourage people to start with manageable goals and to celebrate successes. So if you can get 10 minutes straight a few times a week (if you're coming back from a really rough first trimester or something life crashing like a move...) that's a GREAT START.
The goal would be to just do a little bit more each week, and if you get knocked down from stress, sickness, etc, give yourself "credit" for taking care of yourself - taking vitamins, getting rest, and eating good healthy whole foods.
Another way to get moving or to work with physical activity goals is to strap on a pedometer. I run pedometer walking programs many times a year for many different groups and the goal is 6000-8000 steps a day. You'll probably find that you get 2000 steps normally unless you're on your feet a lot or are chasing a toddler around all day. 2000 is like drive to work, sit, drive home, do some stuff around the house, that's it. Basically you'd up your steps by taking a 45 minute walk.
I myself am a plus sized person who's incredibly healthy. It's kind of hard being in the wellness field but it's been a challenge to my self confidence that I'm happy to say most days I've overcome. It's not about being perfect - it's about being kind to yourself first - making your head a nice place to live. When you act kindly toward yourself and can set a timer for ten minutes and just turn on a CD and dance - you're on your way.
Every little bit counts, celebrate your successes no matter how small. Getting "active" in small steps is one of the best things you can do (as mamachef says) to set an example for your little ones. However no example is a good one if it involves negative self talk.
CHEER TO YOU ALL!!!