Just like in Hawaii, ppl in the DR are on a permanent vacation. They see everything from 24 y/os in string bikinis to 60 y/os in speedos with a big hairy belly. No one is going to find a pregnant woman offensive.
I know a lot can happen in 4 weeks, but it probably won't be anything major. Just get whatever feels comfortable. you can always wear a wrap/sarong/tunic over top for walking around