My wife has a pair of pyjamas that she likes to wear when she is a nidda. She even once remarked whilst wearing them that she was wearing her ‘niddah trousers’.
I have heard in the past that if a woman has a specific item of clothes that she wears when nidda this can make her a nidda if she wears it.
My wife is pregnant and wore these trousers. She certainly had no intention to make herself a nidda when putting them on. We discussed it and she explained that all she meant is that she wears them often whilst niddah as they are comfortable and tznius but in no way does she view them as specifically for when she is a nidda.
Is it ok for her to wear these trousers and are there any ramifications if she wears them?
The idea that they used to wear special nida clothing was because there were numerous halachos of tuma and tahara that everyone had to know. Therefore those clothing were a sort of statement. Nowadays, such clothing are not really applicable unless it is meant to be message between the couple themselves. However as your wife has told you, she is not putting those trousers on as a statement, rather because that it happened to come out numerous times that she wore them when she was a nida. Therefore her wearing them is of no concern at all.