Op never tell anyone what you are wearing again. Just say ''oh im not sure yet'' if they ever ask. friends like that can go get stuffed. wear the dress anyway.
I happen to think that it's always unreasonable and high maintenance to expect to tell other adults how to dress. When I got married I didn't even tell my bridesmaids what to wear, I just told them the colour scheme and let them pick their own dresses. I had one guest turn up in a white dress, she was a friend who was pregnant and I knew they were doing it tough, I assumed it was the only thing she had to wear and I knew she wasn't being spiteful, I was just happy she attended and was comfortable.
I take your point though, everyone is different and not all will share my point of view and obviously some of my friends don't and it's something I have to accept if I want to continue being friends with these people, and for the most part I think I do accept those differences, obviously since I bought a new dress for this wedding and didn't attend my other friends party. I'm only human though and sometimes I get annoyed that they can't extend the same understanding to me that I do for them.
Pregnant for the first-time?
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