I had never thought about altering regular pants into maternity until I saw the movie "Juno," in which Juno's mom alters her jeans to fit her daughter's ever growing belly. Taking a bit from each of them, I referred to instructions and tutorials from Grosgrain, Paisley Princess, and Megan Nielsen. I have just over 2 weeks until my due date and figure I should probably post this now.
Before (and I should have made sure they were a little bit roomy in the butt/hips area since I do get bigger there too - maybe you don't and then just get whatever fits pre-big belly.)
Round one I chose a fabric that was similar to the blue in the denim. It was an old shirt cut up. I didn't make it tall enough in front and probably should have pulled a bit tighter. From the best I could tell, all Kathleen on Grograin did was use fabric.
I liked how Anna from Paisley Princess used a contrasting fabric, so I tried again. So, I got a stretchier tank top at the thrift store that had a shelf bra in it which gave it more elasticity (something I thought would help). I decided to make it a little taller in front since the blue version just settled too low for my comfort. It still was too loose and I decided that I had to use elastic. All my store bough ones had it, so, I unpicked again.
The only tutorial I found that also used elastic (which I will ALWAYS do again!) was Megan Nielsen's. Round three was the most successful by far. I should have started this way.