Many women seem to have low self esteem with themselves when pregnant...as long as they are appropriately dressed and clothed, you shouldn't have any problem with someone taking a selfie then pregnant. If anything, good on them for being confident and showing they are happy.
Now if there was a pregnant woman taking very inappropriate selfies I'd have a problem but I have a friend with a young baby, she took selfies throughout the pregnancy, and you know what? She was happy so who is anyone to judge. Now she has a nice record of her progress through her pregnancy too.