What stage of pregnancy is smoking and drinking the most damaging?

Would it be more dangerous to do these things at the beginning or the end of a pregnancy? I know that fetal alcohol syndrome for example is more damaging during certain months of pregnancy but I do not know which ones.

This is pure curiosity as a biology major.

What is the minimum pregnancy months required for birth?

I have heard that people have been giving births at 7th and 6th months of pregnancy, what are the side effects?