Pregnant Bridesmaid

So my orginal wedding date was May 30th but due to the coronavirus my wedding was moved to August 29th. Yesterday, one of my bridesmaids just informed me she is pregnant and due in october. Since we were planning for a May wedding we have already ordered her dress and unfortunately the dress is discontinued and i cant exchange the dress for a different size...im trying to figure out what to do definitely since Im not even sure if the virus will even allow for an August wedding at this point....if we are able to move forward in August, how to I kindly let her know she would not be able to be in the wedding since I know for a fact the dress will not fit (I plan to still honor her by giving her a corsage and still seat her with the bridal party at the reception) she just will not be in the ceremony...is it rude to replace her with my fiance's niece. I know she would not mind helping out if needed as im really particular about keeping the bridesmaids and groomsmen even. This virus is surely finding was to disrupt our plans!