Congratulations!  😀
First off, let me start out by saying this isn’t an area of my expertise.  I’ve never been pregnant, but I can imagine that the whole notion of getting decent fitting clothing to be really frustrating.  I consulted my mother, who is a master sewer, and she told me that she lived in loose fitting dresses and overalls and avoided dealing with maternity wear.  So anyone with some great maternity clothing wisdom, please feel free to add to this conversation.

That being said, I’m going to use my imagination and think about what I would do in such a situation.  So, if it was me, I would stick to patterns that utilize knit fabrics.  That way it’ll be a little more forgiving by giving you a little extra stretch for when you need it.  In picking out a pattern size, pick the one that best fits your bust measurement.  That way, you don’t have to do any bust alteration.  For most patterns, the front pattern piece is actually half of your front.  (You either cut two out or place once side on the fold of the fabric so when you open it up, you get a whole front piece.)  Okay, so take your front pattern and drape it on the front of your body, so the armhole lines up with your armhole and the center front line goes down the middle of your front.  Don’t forget to take into account that there may be seam allowance built into the pattern so, for example, when you line up the shoulder, you may have to move your pattern up a bit.  It would probably be best to have a friend help.  Ok, so your wearing half of this tissue pattern on your front.  mark on the pattern underneath your bust, also mark your waistline.  At this time, you can also note how short the shirt looks like it’ll be.  So you’ll definitely have to add length to the hemline and write down how many inches you’d like to extend it so you’ll feel comfortable with that length.  Maybe make it a little longer as you can always shorten it, if need be.

Ok, you can take the tissue paper off now.  Tape some extra tissue paper to the side seam (at the bustline you marked) and also the bottom of your front shirt pattern.  Measure your waist from your side to across the front and to the other side.  So you’re not doing a full waist circumference, just the your front.  Take this number and divide it in half.  (I would add a little bit for ease just to play it safe, maybe a 1/2″)  Note the difference between your measurement and the pattern’s waist length.  Extend your pattern’s waistline by making a mark on your tissue paper that’s now taped to the side seam.  (Don’t forget to take into account to add seam allowance.)  Draw a smooth line now from your side seam mark for under your bust out to your new waistline and then continue the line straight down.  Extend the center front line down to how many inches you want to make the shirt longer.  (you’ll have to do this same length extension to the back pattern as well so all the side seam lines are even.)  Make sure your new side seam line extends to the new length line.

After you make these alterations, make a muslin.  This is a test to see if these alterations are even going to work.  If you plan on using knit fabric then your test fabric should also be knit, just pick the cheapest one you can find.  Just cut out and baste the bodice pieces together.  You don’t have to worry about sleeves or anything.  We just want to see how it fits.  Once you try on your muslin, you can make note of where you want to take it in and let it out and transfer these changes to your pattern accordingly.

Phew!  It’s really hard to write about this in a coherent manner.  I hope that isn’t too confusing, and again, this is just how I would go about it.  I hope once you experiment a little, you can come back and give us tips on how it worked best for you.  Good luck!