All I did was add 1/4" elastic to both side seams. I started just below the bust, secured the elastic with a few stitches, and then stretched the elastic as I sewed down the seam using a zig-zag stitch (the more you stretch the elastic, the more gathered the fabric will be). I stopped the elastic about 1/2" above the bottom hem of the shirt.
Here's what it looks like from the inside (not my tidiest work, but oh well):
Very easy! You still have plenty of room for the growing bump, and don't have to wear a tent. Hurray for that.