I think getting as good of nutrition as possible throughout as much of the pregnancy as possible is probably the best. Granted, I have spent several first trimesters just getting by (though thankful for the limited Zofran I took that helped with that a little). However, I think I agree that 2nd-3rd trimesters are the most important, especially 2nd trimester. That is, also, of course, if you do your best in the first trimester and went into it with good reserves. The baby WILL take what it needs from mother, but that can set mom up for depletion in later pregnancy with teeth and bones are really forming well, which I don't think would be good.
Just as examples from my own pregnancies - first one has a more pinched face, teeth spacing issues from toddlerhood, and some behavioral issues - SAD the whole pregnancy.
2nd was Brewer pregnancy diet - he has a nice wide face, great teeth, huge wide feet, still some behavioral issues, but mainly it's personality
3rd modified Brewer diet (so he wouldn't be 10.5# like his older brother) - also nice wide face with good teeth, good feet, etc.
5th more "healthy" SAD than Brewer, but still fairly decent - so far, he looks like he has good facial/jaw/teeth structures and nice feet.
As I had/have more pregnancies, the first trimester gets harder and harder. But I try to make sure I'm eating well the rest of the pregnancy, and so far, those little faces look good. I'm a little concerned about this little guy, only because I really slipped into SAD out of desperation for not feeling well, until about 31ish weeks when we switched over to PB/paleo.
Anyway, long way to say that I think that doing your best, and focusing on 2nd-3rd trimester (and beyond) is probably the best way to go, if you can.