do it! i just got one on my belly a week ago as part of my belly blessing (I'm 37+4 today), and it is stunning! two of my friends who had them done said that they lasted about 2 weeks, then, poof! were gone.
who cares what it looks like after birth? for now your belly will be even more beautiful! And, you have stretch marks like me, right? If I had known about henna w/ DS1 (when I got the suckers), I would have had henna all the time!
the artist who did mine said that once the henna paste peels off, it will take about 24 -48 hours to get as dark as it's going to. This depends on your skin type and texture, but mine is still pretty distinct. (when this is done for say, an east indian wedding, they do the palms b/c coarser, thicker skin takes the dye more readily). Then, it will gradually wear away. moisturizing your skin (what belly doesn't love that?) is supposed to help too.