It's been said, but 3-6 weeks is so early in pregnancy as to not even be a consideration in terms of SN dosage.
Having said that.
19 is a perfect age to become a mother, why wouldn't it be? There is nothing, at all, wrong with young motherhood. The social stigma attached to teen pregnancy is very antiquated, prudish, and out of touch with the biological reality of these things. At 19 you are in your childbearing prime. I became a mother at 21 myself, and in fact now I wish I had started a few years younger. So, I'm actually a little bit jealous of your opportunity, truth be told.
By no means am I trying to impose if your mind is made up, I respect that. I just have a difficult time understanding what is so wrong with your circumstances. I want you to know, how many very depressed and unhappy, 30 and 40 something year old women there are out there who wasted their childbearing prime because they "weren't ready". Looking back in hindsight, I'm sure many of them wish they could have made the "mistake" of being in the situation you are in right now.
Being pregnant is a natural antidepressant, the high estrogen levels have a noticeable mood boosting effect. I can say honestly, the times I have been pregnant have been the happiest times in my life, whatever else was wrong in my life at the time didn't matter, because I was pregnant.
I wish you all the best regardless of what you choose, we're all here for you in any case.