These feelings you describe are common during pregnancy. I can't type much now but know that you aren't bad and aren't alone. 

I've been in the same position. I would retreat into my room when SSs were around because I couldn't stand the tantrums and the drama. This was before I was pregnant, so I can't blame hormones. Now that I have two little ones, I'm even less interested in the tantrums and the drama (that are still happening with two 17 year old stepsons). 

I also feel even more resentment towards them, because my children are constantly being pushed aside or expected to accommodate their teen brothers even though they are a toddler and an infant. Not to mention the fact that we found out this weekend that BM is actually making more than both DH and I combined, but DH is still paying 1/4 of his income to her each month in CS. We found this out because one SS decided that DH should start paying the CS directly to him each month because his mom doesn't need it, but doesn't "give him" any of the money. His mom gives him plenty of money, so I'm not sure why he's complaining. Paying CS is currently a financial hardship for us and DH is paying that amount because BM lied about her income at the last mediation and DH's lawyer was too crappy to pursue it further (she told DH he could always file for an adjustment in two years). It's hard not to feel resentment towards all this. It's not totally SSs' fault in my case, but they are now old enough to see BM for who she is (and at least one of the does), but they still serve as her spies in our house and still let her use them as tools to get to DH. 

I totally understand the way you feel. Our kids age gap was the same and I found ss to be the most annoying brat ever. I could barely look or speak to him during pregnancy. Sadly it got worse after as he would try and goad baby into dangerous situations and I would loose my cool.

Honestly though, in hindsight I think he was looking for attention and I was doing the exact opposite trying to escape him. I wish I'd have been a bit kinder and more understanding at that time. And just in my case the kids love each other to bits now and are so alike in many ways. So after all the years of grief it's now a joy to watch them together... sometimes ! Lol

As i wrote in reply to another poster recently who was having feelings similar to yours, i think there may be some kind of hormonal aversion to unrelated young kids that happens during pregnancy. When i was pregnant with my first, i was young, had no stepkids, and really no emotional baggage related to children at all. Even with that, i found myself trying to stay away from other little kids. Not teens, but young kids. I had this feeling that they were germy and a threat to my unborn child. Upon learning more, there are certain childhood illnesses that, if caught for the first time while pregnant, that can harm a developing fetus. Perhaps this is some evolutionary process that helps protect pregnant women, as unrelated kids would be more likely to carry diseases that a woman hasn't been exposed to before, as opposed to kids she is related to and has been around quite a bit.

Now, your stepchild has likely been around you a lot, so i don't think they are an actual threat to you. I think it is probably hormone-related. I would recommend talking to your doctor about it, and maybe together you can speak to your husband about giving you a break while you are pregnant and for a while after the birth. I don't know how much childcare you do when your SS is present. "Dear" husbands have a tendency to think that women just love taking care of kids so they may put a lot of childcare on their wives. If this is happening, it needs to stop. If your DH does all the childcare, is it that you don't have boundaries in your home? When i first started dating my SO, there were no boundaries. Skids were allowed into his bedroom and bathroom whenever they wanted to be. I had to set the boundary because SS saw me naked several times. Also, you have to have a place in your home where there is peace and quiet. Blame your pregnancy. Tell your DH that you can't handle any extra "stress" right now. After all, it's the truth.