I made this top using this free pattern from Burda Style. I gave it a placket opening in the front for future nursing access although I am unsure about how the thin material will stand up to the messes of motherhood. There was no need to alter the pattern to fit a pregnant belly really.... and I am 33 weeks. I did add a seam in the front where the elastic is for the placket and the elastic does not continue to the back as in the original pattern.
The button function on my Pfaff Hobby works really well but it was a bit nerve wracking to do. They did not end up perfectly straight but hopefully you can't really tell.